How to introduce new fish to an aquarium without killing old ones?

If the infection is cleared, wait a couple weeks and then consider repopulating the tank. Ideally, you should set up a quarantine tank for incoming fish before they go into the main tank. Monitor them for a week or two to confirm good health. You may even want to administer an antiparasitic medication while in quarantine to be safe.

The majority of the acclimation guide is intended for introducing animals recently purchased online into a quarantine environment that have been inside a dark box for 24+ hours and the water quality is most likely degraded from waste. Hopefully you followed these steps before introducing your pond animals into their quarantine environment. After the quarantine period the acclimation from this environment should be relatively easy and quick – given the water parameters in the quarantine environment are ideal and similar to your pond. The best way to move the fish would be to put them in a 5 gal. bucket and float the bucket in the pond for at least 30 minutes so the temperature in the bucket can gradually reach the same temperature as the pond. Make sure you do this in a shady spot of the pond or during a time period when the bucket will not be in the sun. In order to keep the bucket from tipping it may take some creativity but it may be as simple as using a wooden pole laid across the pond centered through the bucket handle to achieve this. Make sure to add a small air-stone to the bucket to agitate the surface so the oxygen and pH levels don’t change. From there you can gradually introduce pond water to the bucket and remove the mixed water until you are confident that the majority of the water left in the bucket is water from your pond (should take no more than 20-30 minutes). From there, gently scoop the fish out and place them in the pond and discard the water from the bucket.

The most common cause of death of a newly acquired fish is usually shock or incorrect water quality. Fish tanks should be set up and left running without fish for at least 3-4 weeks to establish a proper nitrogen cycle. There are products that can shorten this time period. If this is not done, there may be high levels of ammonia, chloramine, nitrates and nitrites which can kill a fish. When introducing a fish, the bag should be floated in the tank for about 30 minutes to acclimate the fish to water temperature. This can prevent shock if the water is too hot or too cold. Test your water quality with a home test kit or submit a sample for testing. Most pet stores or aquarists will do this for free or for a nominal fee. Correct any imbalances as indicated. Slowly acclimate any new fish as written above. You can request a consultation to discuss your concerns in greater detail if necessary. Buying fish and starting a tank can be quite confusing initially.

It is possible the tank has not cycled properly which results in high ammonia and nitrite levels which can kill fish. Allow the tank to run, without fish, for at least 4-6 weeks. During this time, use a home test kit or submit a water sample for testing every few days. The local Petco may do this for free or for a small fee. What you are looking for is to have ammonia and nitrites at 0 ppm and nitrates at 20ppm or less. pH can hover around 7 for most community fish. Avoid large, rapid swings in pH though. What is happening in the tank during this time is that beneficial bacteria are populating the gravel/substrate and the filter. These bacterial populations are responsible for the Nitrogen Cycle. This cycle breaks down wastes from fish, uneaten food and plant matter into by-products that are filtered out or dissipate. You can "cycle" a tank with fish, however this increases the risk of loss. Once your tank has cycled, you can add fish. You will still need to monitor water chemistry and perform partial water changes regularly. Use only conditioned water that is temperature matched to prevent stress or shock. You will also need to vacuum the gravel regularly and rinse the filter media. Never use tap water to rinse. Use a bit of tank water to protect bacterial populations in the sponge.

Thanks for reaching out about your arowana. These are tough fish to co-habitat in an aquarium as they tend to eat other fish and can be territorial. Introduced fish must be large enough to not appear as prey and not too timid, or will hide and not compete for food. This readily depend on how large your arowana is at present. Good luck.

Ideally, the tank should be kept at about 86 F for two weeks to resolve Ich infection. The gravel or substrate should be vacuumed and tank furniture sanitized. If you are setting up the tank anew, you will have to allow time for it to properly cycle prior to adding fish.

Ich (Cryptocaryon) is an obligate parasite. This means it needs fish to survive. If your tank has no fish, continue to run the filter and tank empty for at least 6 weeks. You can increase temp to about 86 F to speed up the life cycle of the parasite. If your tank is not a reef tank and has no inverts, you can treat the tank with copper. Once treatment is complete, decrease temps to normal ranges. Continue to monitor salinity/chemistry. Quarantine any new fish for at least 10 days in a bare bottomed, established tank to prevent re-introduction of parasites into the main tank.