How to Add New Fish to Your Aquarium: Quarantine and Acclimation Methods

Adding new fish is more than just floating a bag and tipping it in. Success comes from thoughtful preparation, a proper quarantine tank setup, and choosing the right acclimation method for the type of fish you’re bringing home. Whether you keep community tetras, sensitive dwarf shrimp, wild-caught cichlids, or marine species, the process centers on three goals: avoid disease introduction, reduce stress, and ensure a smooth transition to your water parameters.

Also Read-Aquarium Water Parameters Explained: pH, Hardness, and Nitrates

This guide will walk you through the essential steps to ensure your new arrivals—and your existing community—thrive. We’ll cover the non-negotiable step of quarantine and detail two proven methods to acclimate fish, ensuring a smooth transition from the store to their new home.

The Quarantine Tank:

Before we even think about acclimation, we must discuss the single most important step in adding new fish: quarantine. Think of a quarantine tank as a reception area for your new fish. It’s a separate, smaller tank where new arrivals live for 4-6 weeks before they meet your established pets.

Why Quarantine Matters.

Quarantine prevents diseases, parasites, and pests from reaching your display tank. Even reputable stores and responsible breeders can unwittingly sell fish that carry ich, velvet, internal parasites, bacterial infections, or hitchhikers (especially with plants and inverts).

• Disease Prevention: Fish from stores can carry parasites, bacteria, or fungi that aren’t immediately visible. Ich (Ichthyophthirius multifiliis), Velvet (Oodinium), and bacterial infections can hide on a seemingly healthy fish and wipe out your entire main tank. Quarantine allows you to identify and treat these issues in isolation.
• Stress Reduction: The journey from breeder to store to your home is incredibly stressful for a fish. A quiet quarantine tank allows them to recover, eat without competition, and regain their strength.
• Observation: This period is your chance to observe the new fish’s behaviour. Is it eating well? Is it swimming normally? Any subtle signs of illness can be caught and treated before it ever reaches your display tank.

For most freshwater and marine setups, a 2–4 week quarantine is the sweet spot. Two weeks is the bare minimum; four weeks gives you time to observe multiple feeding cycles and parasite life stages.

Quarantine Tank Setup.

A good quarantine tank (QT) is simple, stable, and easy to sanitize. You do not need an elaborate system; you need dependable basics that help fish destress.

• Tank size: 10–20 gallons suits most community fish. Scale up for larger or more numerous fish.
• Filtration: A seasoned sponge filter is ideal. Keep one running in your display tank so it’s ready to move into the QT when needed. Add an airstone for robust oxygenation.
• Heater and thermometer: Match the species’ preferred range and maintain steady temperature. Use a reliable heater guard for bottom sitters like loaches and catfish.
• Bare bottom: Easier to clean and monitor waste. Use a light-colored bottom if you want to keep an eye on feces and parasites.
• Hiding spots: PVC elbows, inert plastic plants, or ceramic caves give security without harboring medication-absorbing residues.
• Lid: Many fish jump when stressed. A tight lid prevents heartbreak.
• Lighting: Keep it dim. Bright light raises stress.
• Test kit and water change tools: Check ammonia, nitrite, nitrate, and pH. Have a dedicated net, siphon, and bucket for QT to avoid cross-contamination.
• Optional: Background or side coverings to reduce visual stress; an ammonia alert badge for quick checks; a piece of floating plant (in freshwater QT) for cover.

Important note about medications: Two approaches exist. Some aquarists treat prophylactically (e.g., anti-parasitics) during QT; others observe first and only treat if symptoms appear. If you keep invertebrates, avoid copper and formalin. Whichever path you choose, document doses and watch closely for adverse reactions, especially with scaleless fish (loaches, Corydoras, plecos, some catfish, puffers) that are more sensitive to meds and salt.

Preparation Before You Buy.

Success starts before you bring the bag home.

• Test your display tank’s parameters and ensure they are stable: temperature, pH, hardness, ammonia, nitrite, nitrate, salinity for marine tanks.
• Prepare your QT with matched parameters and fully heated, aerated water.
• Seed your QT filter media in your main tank at least a week prior (ideally longer).
• Gather acclimation tools: airline tubing, a clip or valve to control drip, a clean bucket, water conditioner that neutralizes ammonia (e.g., a binder), a flashlight for fish inspection, and a soft net.
• Plan transport: Ask the shop for double-bagging with oxygen, minimal water for shorter trips (reduces sloshing), and a dark bag. Avoid buying fish on the store’s delivery day; allow them to settle at the store first.

Acclimation Basics: Why It Matters.

To acclimate fish is to ease them from store or shipping water into your water. Sudden changes in temperature, pH, hardness, salinity, or dissolved gases can shock fish and inverts, even if they looked fine in the bag. At the same time, lingering too long in bag water after opening can be dangerous because ammonia accumulates during transport and becomes more toxic as pH rises with aeration.

Key principles:

• Keep acclimation efficient, not endless. Aim to finish within 30–90 minutes in most cases.
• Never add store/shipping water to your tanks. Net fish out and discard the bag water.
• Use an ammonia binder in the acclimation container if the trip was long or if fish appear stressed or the water smells pungent.
• Dim lights and reduce noise to lower stress.

Choosing an Acclimation Method.

There are two reliable ways to acclimate fish: the floating bag method and the drip acclimation method. Both work well when used appropriately.

Floating Bag Method: Step-by-Step.

The floating bag technique is efficient and works well for hardy species and short trips. It’s also a solid choice when you suspect ammonia has built up, because it keeps total acclimation time short.

1. Lights off and room dim. Turn off aquarium lights in QT and display. Darkness reduces cortisol-driven stress responses.
2. Float the bag 15–20 minutes. Place the sealed bag in the QT to equalize temperature. Do not open yet. Keep it away from strong light or flow.
3. Prepare an acclimation container. After temperature equalization, open the bag and gently pour the fish and bag water into a clean bucket or specimen container. Optionally add an ammonia binder at the label dose.
4. Add QT water gradually. Every 5 minutes, add a small cup of QT water (about 10–15% of the current volume). Over 20–30 minutes, you should double the volume, slowly mixing your water with the bag water. Watch the fish’s breathing and posture. If signs of distress appear (rapid gilling, loss of equilibrium), shorten the process.
5. Transfer the fish. Use a soft net to move fish into the QT. Do not pour acclimation water into your tank. Discard it.
6. Oxygen and cover. Make sure the QT has good surface agitation. Keep the lid closed and lights low for several hours.
7. First feeding. Wait to feed until the fish settle (often the next day). An exception is tiny species with high metabolisms; offer a small, easily digestible meal (e.g., a few crushed flakes or live baby brine) after several hours if they appear active.

When it’s best:

• Community fish raised in water similar to yours.
• Hardy fish like many livebearers, danios, rainbowfish, most captive-bred cichlids.
• Situations where bag water likely has elevated ammonia and you want to minimize time in it.

Drip Acclimation Method: Step-by-Step.

Drip acclimation shines for delicate species and invertebrates or when your water chemistry differs from the source. It’s especially valuable for shrimp, snails, otocinclus, wild-caught fish, and marine reef inhabitants.

1. Lights off and room dim. As above, keep stress low.
2. Set up your drip. Place fish and bag water into a shallow bucket or specimen container. Secure an airline tube from the QT to the bucket. Start a siphon and tie a series of loose knots, or use a drip valve, to control flow. Aim for 2–4 drips per second.
3. Use an ammonia binder. Add per directions if transport exceeded an hour or if you detect a strong odor. This reduces toxicity as pH stabilizes.
4. Drip to double volume. Over 45–60 minutes, allow your water to drip until the volume in the bucket roughly doubles. For marine fish/inverts, consider measuring salinity to confirm a gentle rise to your target.
5. Remove half the water and repeat (optional for very sensitive species). Discard half and continue dripping to double again for an extra-gentle transition. Total time 60–90 minutes.
6. Maintain temperature and oxygen. Place the bucket near the QT heater to avoid chilling. Point a small fan across the surface or drop in an airstone for oxygenation (avoid vigorous bubbling that splashes).
7. Transfer with a net. Move fish into the QT, discarding acclimation water. For shrimp and tiny fish, use a specimen cup or fine-mesh net.
8. Cover and keep it quiet. Lid on, lights low, minimal movement around the tank for several hours.

When it’s best:

• Delicate or scaleless species: otocinclus, Corydoras, loaches, puffers, certain plecos, discus, and many wild-caught fishes.
• Invertebrates: dwarf shrimp, snails, crabs, marine corals and inverts.
• Marine fish in general, especially when salinity or pH differs from the seller.
• Anytime your pH, hardness, TDS, or salinity meaningfully differs from store parameters.

Safety tips:

• Do not exceed about 2 hours total. Long acclimations risk cooling and re-toxifying ammonia as pH rises.
• If fish show distress during dripping, increase drip rate slightly or shorten the process.
• Always discard acclimation water.

Special Case: Shipped Fish and High Ammonia.

Long-distance shipments often arrive with low pH and high total ammonia. When you open the bag, pH rises and un-ionized ammonia becomes more toxic. To protect fish:

• Add an ammonia binder to the acclimation container immediately after opening.
• Keep the process efficient: float for temperature, then choose a short floating method or a brisk drip (30–45 minutes) depending on species sensitivity.
• For extremely stressed fish, prioritize getting them into clean, warm, oxygenated, matched-parameter QT water quickly. The safer path is often the shorter one.

First 72 Hours in Quarantine.

The transition is not over once the fish leave the bucket. What you do next can mean the difference between a thriving addition and a setback.

• Keep lights dim for the first day. Use a normal photoperiod on day two or three.
• Observe breathing rate, swimming posture, appetite, and feces. Look for white stringy feces (possible internal parasites), flashing, clamped fins, frayed edges, or spots.
• Feed lightly. Small, frequent meals are better than a feast. Offer high-quality, easily digestible foods: soaked pellets, small frozen foods (daphnia, brine shrimp), live foods to jumpstart appetite.
• Test daily. Ammonia and nitrite should be zero; nitrate <20–40 ppm in freshwater; maintain stable salinity in marine tanks. Perform a 25–50% water change if ammonia or nitrite appear.
• Consider prophylactic internal parasite treatment if the species is prone (e.g., wild-caught fish) and if you prefer proactive care. Always research species-specific sensitivities.

New Fish Hiding: What’s Normal and What’s Not.

It is common to see new fish hiding. Stress from transport, bright lights, unfamiliar surroundings, and new tankmates all drive hiding behaviors. Knowing when to worry—and how to encourage exploration—will help.

Normal hiding:

• The first 24–72 hours, especially in bright or busy rooms.
• Shy or crepuscular species (e.g., many catfish, loaches, dwarf cichlids).
• Fish without adequate cover.

How to reduce new fish hiding:

• Add cover. Use caves, plants, driftwood, and line-of-sight breaks. Even PVC elbows work in QT.
• Dim the lights and add floating plants (in freshwater) to soften light and provide security.
• Use dither fish in community setups. Active, peaceful schooling species signal safety to shy fish.
• Feed at dusk. Offer small portions when lights are low. Place food near cover so shy fish can eat without venturing far.
• Minimize traffic and noise around the tank during the first few days.
• Keep parameters steady. Fluctuations exacerbate stress and hiding.

When to worry:

• No interest in food after 72 hours (some species and wild-caught fish can take longer, but watch closely).
• Rapid breathing, gasping at the surface, lying on the bottom or floating near the top without control.
• Visible lesions, cottony growths, ich spots, ragged fins, or emaciation.

What to try:

• Offer live or frozen foods to trigger feeding. Live baby brine shrimp or blackworms often tempt reluctant eaters.
• Soak dry foods briefly to improve palatability and reduce air swallowing.
• Provide species-appropriate diet: algae wafers for otos and plecos, small meaty foods for micro-predators, gel foods for grazers.
• Add more cover and reduce light further for the most timid fish.

Integrating from QT to Display.

Once your fish have completed quarantine symptom-free, bring them into the display with care.

• Match parameters. Temperature, pH, hardness, and salinity (if marine) should closely match the display’s water.
• Rearrange decor before release. Breaking line-of-sight and territories reduces aggression from established tankmates.
• Use an acclimation box. For semi-aggressive communities or cichlid tanks, an acclimation box allows the group to see and smell the newcomer without contact for a day or two.
• Release after lights out. Night releases reduce aggression and stress.
• Feed the display tank generously—but not excessively—right before release to keep resident fish busy.

Common Mistakes to Avoid.

• Skipping quarantine. This is the number one cause of tank-wide issues after new arrivals.
• Pouring bag water into the tank. This risks introducing pathogens, copper, methylene blue, or antibiotic residues.
• Overlong acclimation. Prolonged exposure to bag water can increase ammonia toxicity.
• Not using an ammonia binder when needed. Especially with shipped fish.
• Inadequate biofiltration. An uncycled QT leads to ammonia spikes. Pre-seed that sponge filter.
• Overmedicating or mixing meds rashly. Sensitive species can crash under heavy treatment. Go slow and dose precisely.
• Adding too many fish at once. Your biofilter and your ability to observe both suffer when you overload.
• Bright lights and no hides. Stress escalates; “new fish hiding” becomes chronic.

Species-Specific Notes.

• Livebearers (guppies, platies, mollies): Generally hardy; floating bag method works well. Watch for columnaris and internal parasites; quarantine at least two weeks.
• Tetras and rasboras: Most captive-bred are hardy; floating bag usually fine. Wild-caught characins benefit from drip acclimation and very clean QT water.
• Corydoras and loaches: Scaleless; prefer drip acclimation and gentler med approaches. Provide soft substrate or bare bottom with smooth hides.
• Otocinclus: Very sensitive. Drip acclimation advised. Offer seasoned surfaces, blanched veggies, or quality algae wafers in QT.
• Dwarf cichlids (Apistogramma, ram cichlids): Sensitive to parameter swings; drip acclimation recommended. Provide caves and leaf litter for security.
• African cichlids: Generally robust but territorial. Floating bag method is fine; ensure similar hardness/alkalinity. Add multiple hides to reduce aggression later.
• Gouramis and bettas: Sensitive labyrinth fishes appreciate gentle acclimation and calm, warm QT with low flow.
• Marine fish and inverts: Drip acclimation is standard. Match salinity carefully. Be aware that some LFS systems run copper; never mix store water into reef tanks.

Practical Checklists.

Arrival day:

• Lights off, room dim.
• Float bag 15–20 minutes.
• Set up acclimation container, binder ready.
• Choose floating bag or drip method based on species and parameter differences.
• Net fish into QT; discard acclimation water.
• Lid on, gentle aeration, low light.

During quarantine:

• Test water daily at first.
• Feed lightly, focus on quality.
• Observe behavior and appearance closely.
• Treat only as needed or follow a carefully planned prophylactic regimen.
• Perform 25–50% water changes 2–3 times weekly, adjusted to bioload.

Display day:

• Match parameters.
• Rearrange decor, use acclimation box if needed.
• Release after lights out.
• Monitor for aggression and appetite.

Frequently Asked Questions.

How long should I quarantine new fish?
Two to four weeks is recommended. Two weeks minimum for healthy, captive-bred fish; four weeks for wild-caught or delicate species.

Should I always drip acclimate?
No. Drip acclimation is best when your water differs significantly or for sensitive species and inverts. For hardy fish with short transport times, the floating bag method is faster and often safer due to less time in ammonia-laden water.

Can I quarantine multiple species together?
Yes, if they are compatible in temperature and temperament. Avoid mixing aggressive fish with timid ones in QT, and do not medicate the group unless all can tolerate the chosen treatment.

What if my new fish won’t eat?
It is common for new fish hiding to ignore food for a day or two. Try live or frozen foods at dusk, add cover, reduce light, and keep water pristine. If there’s no feeding after 72 hours and signs of stress appear, reassess and consider targeted treatment.

What about plants and invertebrates?
Plants can carry snails, algae, and pathogens. Consider a plant-only quarantine or dip. Inverts require invert-safe QT protocols; never expose them to copper unless intended and species-safe.

Bringing It All Together.

If you’ve searched how to add new fish and found conflicting advice, remember that your goals are simple: keep pathogens out, reduce stress, and match parameters thoughtfully. A basic quarantine tank setup lets you control conditions and observe without risk to your display. Select your acclimation method to suit the species and situation—floating bag for hardy, short-transport fish; drip acclimation for sensitive species, invertebrates, and significant parameter differences. Manage the first 72 hours carefully, expect some new fish hiding, and respond with cover, calm, and clean water. Quarantine for 2–4 weeks, integrate thoughtfully, and you’ll stack the odds in favor of long-term health and a peaceful community.