When is it safe to put ich-treated fish back in the main tank?

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.

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.

Increase the temp, gradually, to 86 F and hold it there for at least two weeks. This will speed up the ich lifecycle. When the parasite is free swimming is when it is most susceptible to treatment. Once the spots drop off your fish, perform a major water change and vacuum the substrate thoroughly. If you have live plants, remove them and gently wipe them down, if you have artificial plants, remove them and sanitize them. The same goes for any tank furniture. Use the ich medication according to label directions. Keep the water temp elevated for 3-5 days after treatment and after all symptoms have resolved. Gradually reduce water temp back to normal afterwards. There is no definitive way to determine if the infection is resolved. You will to monitor the fish closely and repeat the process if necessary.

When there is an ich infection, it is necessary to treat the entire tank, even the fish that are not symptomatic. Ich has 3 distinct stages in the life cycle. Tomites, the free swimming stage, are most vulnerable to treatment. Once on the fish or encapsulated in the substrate, ich is very difficult to treat. A combination of increased water temp, aquarium salt and commercial medication can be used to treat ich. The increased water temp, 86F, will significantly speed up the lifecycle. It can take 1-2 weeks to treat minor infections and longer for more severe infections.

The safest treatment would be a temporary increase in water temperature and some plain aquarium salt (NOT table salt). Ich is very sensitive to temperature. Gradually increase water temperature, by no more than 1 degree F per hour, up to 86 F. Hold it at that temperature for at least 2weeks. This will speed up the life cycle and prevent reproduction. You can perform partial water changes (30%) and vacuum the substrate during treatment. Use only aged or conditioned water that is temperature matched to prevent stress or shock. An airstone should be used to increase oxygenation since warm water does not hold as much oxygen as cooler water. This could affect some the fish. Aquarium salt can be dissolved into the tank at 1-2 teaspoons per gallon of water. Dissolve it in a cup of tank water then pour it gently into the tank. When performing water changes, treat only the amount of water being replaced with salt, not the entire tank volume. After 2 weeks, gradually decrease water temperature back down to species appropriate levels.

Gradually increase the water temp to about 80 F. This will speed up the lifecycle of the parasite. Once all the spots have dropped off the fish, you can use a commercial ich treatment. Look for one in the aquarium section of the local Petco. Follow label directions carefully. If you have just a few fish in the tank, you can remove them to a separate tank after the spots have dropped off. Leave the main tank empty for 4 days at 80 F. The ich parasite cannot survive without fish, so they will die. After 4 days, perform a partial water change, vacuum the substrate, wipe down tank furniture and return the water temp to normal. Acclimate the fish to water temp before returning them to the water.

Even though a picture of it would help to determine underlying cause more precisely, ich can only be killed in its free swimming stage. After it feeds off the fish, it drops into the substrate where it forms cysts and multiplies. Then the new ich parasites swim out of the gravel and find new fish hosts and start the cycle all over again. That is why it is important after all white spots have disappeared to continue treatment, you should continue treating for an additional 2 weeks after all symptoms are gone. After the spots disappear off the fish, I would vacuum the gravel to suck up any cysts. If you increase the temperature the life cycle of the ich becomes faster. All new fish should be placed in a quarantine tank for at least 2-3 weeks to make sure they are eating, free of disease, and are able to recuperate in a stress-free environment.