Spidercat outlined some good points, since the Ichthyophthirius multifiliis parasite is a very popular issue in aquariums i feel like adding my personal experiences as well. **Causes for an outbreak:** > This can be but not limited to **temperatures**, **PH** and **nitrite > / nitrate levels**, **lighting**, **overpopulation!!!** - environmental changes of all sorts. > Anything that causes stress or a weak immune system of the fishes. **Avoiding an outbreak:** > **AVOID STRESS AT ALL COST** - when settling new fishes make sure the water temperature and PH levels don't cause a shock - its highly > suggested to measure tank temperature and PH levels and get > information from the dealer about the water they use - otherwise > measure on your own. If there are big differences use a second tank to > bring the fish slowly to the level of your tank. > > When settling in new fish i cant underline this enough, use a second > tank for 2-4 weeks - specially if the new fishes are weak, tiny, unhealthy, - in most cases the "new fish" will get the > parasite first, this in most cases is not caused by the fish > themselves but by a weakened immune system from the transport from > the farm or a shock from the difference of water quality. > > High nitrite and nitrate levels can harm the immune system of your > fish and accelerate an outbreak. > > Again, having a second tank with 30%-100% of the main tank water > volume can be an immense help to either quarantine fish or to have > "good water" at hand - so you can if in need settle fresh water over > night and realize 90% water changes (ill write about this below). > > **Lets have a look on the life-cycle of the Ichthyophthirius multifiliis parasite.** > The white points / dots on the fish are the **trophozoites** - > normally the first trophozoites are spotted on the fins. Once the > parasite has gathered enough food he will release from the fish and > enter the tomont dividing stage. The tomonts gravitate to the ground > but can stick to any ornamental object. The tomont divides up to 10 > times - within 7 hours each tomont can divide in 1024 encapsuled > theronts which attack the fish - from the the lifecycle repeats itself. > > The parasite lifecycle is dependent on the temperature - wikipedia > states that at 25°C /77°F the lifecycle takes 7 days while at 6°C/43°F > it takes 8 weeks. > > **The parasite does not like temperatures above 30°C and PH below 5,6 and above 9,2 - unfortunately, > most fish cant stand that temperature and those PH values too - some fishcan stand PH values below ~6 which also might be a help.** **My fishes got the parasite, what can i do?** *First and foremost, analyze the mistakes that lead to the spreading of the parasite - otherwise you will sooner or later face the same problem again.* There is no ultimate solution but a few points that certainly will, could help. > - If your fish can withstand it heat up the tank to 30°C - if not, proceed to 2. > > - Make drastic water changes - 90% are reasonable (Here is why a second tank with prepared water can be helpful - you can resettle the > fish every day in the prepared water or just change the water of the > tank. (AGAIN MAKE SURE THE FISH DONT HAVE TOO MUCH STRESS) > > - Get vitamins and artemia - your fish need to stay strong. > > - If possible seperate the infected fish from the healthy fish (again a second or 3rd tank for this purpose is helpful > > - Get an UV-C clearer - in this case bigger / more is better - got multiple filters, put it on each filter, maybe even get one intern > UV-C clearer - the more water you can clear from the parasite per hour > the better. > > - AVOID STRESS at all cost - if the tank is in a fequented room put a towel on it. > > - I did manage to clean fish from the parasite by grinding along wit ha finger (only works if you are really careful, otherwise you do more > bad than good) > > - Clean all plant and other inventory during the sterilization process multiple times. > > - Clean the gravel during the sterilization process with a gravel cleaner > - best to do this before the daily 90% water change - same > for the plants and other inventory. > > - Diatom or sand filter (i haven't tested this yet). > - Lower PH (BE VERY CAREFUL HERE UNLESS YOU REALLY KNOW WHAT YOU DO SKIP THIS) > - medication Final words: If there is an outbreak clearly something went wrong. Tanks are a biologic environment where every change can put it out of balance. Fish dealers are not always experts, often they are great salesman. Its a huge commodity to have a fishdealer who loves aquaristics and is not just doing his job as salesman. I do aquaristics for 22 years, the best advice i got was online and from books in 90% of times. Reading about water and fish will prevent such problems as ichthyo. Last but not least, aquaristics is a serious hobby which needs dedication, passion and some financial efforts to have a longlasting positive experience - the reward if those are put in is awesome - i love my hobby :).