fish in community tank dying

Question
i have had my tank with freshwater fish set up for at least four months now and nothing ever happened till recently. I ended up buying some dwarf cory cats and about a weak later there were little white cotton looking things all over in my tank. at first i thought they may have been eggs layed by the cories so i took the ones i found and put them in a breeding net and set it in my 30 gallon tank. ever since then more of those things showed up and then one by one my fish mysteriously began dying- almost every morning there is another dead. and all my cory cats have died yet i am still finding the white cotton like things in my tank, so i am sure they are not cory eggs. since my fish have been dying, i have been constantly cleaning the tank so i dont contaminate the rest of my fish. i also bought some stuff for ick and parasites but they still seem to die. what should i do, i think that the other remaining fish are in some way sickly too but i dont know whats wrong and i dont know what to do. what do you think the white cotton like things are? if you have any idea i would be very grateful to know whether or not my fish have a chance still.
thank you

Answer
Hi Kerri;

The fuzzy things are a saprolegnia fungus attacking decaying food and/or waste. They might even be attacking old dead fish eggs. That fungus can invade the gills of the fish and kill them if there has been injury or weakness in the gills. The cories were easily attacked because they are on the bottom closer to it. Do some gravel vacuuming and cut back on feeding. Replace 25% of the water every two or three days while vacuuming the gravel until the stuff stops appearing and the gravel is clean. This should slowly get things back to normal without shocking the fish. You don't want to empty the tank and start over. It will have to go through the break-in period all over again and with a fully populated tank it would be more disastrous.

Since the fish have been ill already they may be infected by the saprolegnia that has been floating around in there. If their gills were damaged from ammonia due to overfeeding, the saprolegnia will attack the damaged areas inside the gills. You might want to get your water tested for the presence of ammonia and nitrite. To help kill the saprolegnia, use a medication called "malachite green". It should be straight malachite green, not a medicine that has it as part of it's formula. Also get some aquarium salt. Add the salt at a rate of 1/2 teaspoon per gallon of water. This low dose will not harm any of your fish. I've used it even with cory cats and it is fine. The salt helps the fish heal and gives them back needed electrolytes lost from stress. Use the malachite green according to the directions on the bottle. That stuff stains things so be very careful not to get it on your clothing, furniture, etc. It will stain your fingers too but will wear off in a few days. Use a folded paper towel to hold the bottle while dosing and it does help. Here is a good article about Saprolegnia to help you know more about it;

http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=20709&cid=3788&search=

When you make the water changes, add back enough salt to treat the new water only. For instance, since you will have already dosed the whole tank, treat each gallon replaced at water changes with another 1/2 teaspoon of salt. If you replace 10 gallons, add back 5 teaspoons of salt. Continue doing this until the fish are well again. Once the crisis is over, stop adding salt with your normal weekly 25% water change and the salt will slowly go away.

I hope they feel better soon......

At Your Service;
Chris Robbins