Losing what appears to be healthy fish?????

[kayakers2]

We have a 25 gal. tank with mostly mollies. The water has always tested perfect for GH, KH, PH, and shows no ammonia. We had a guppy disappear overnight a few weeks ago. I took the tank apart (we use the cuttings from our 75 gal. planted to decorate) washed everything, cleaned the gravel, changed the filter media and have done several water changes. All fish appear to be healthy. They eat and are active. Then when we get up in the morning or come home from work there will be a dead fish. When I got home yesterday it was a pregnant mollie dead. This morning a female puffer mollie. Only one fish out of five we lost showed any signs of illness. That was a large male sailfin mollie that developed a white fuzz on his left side. We set up our hospital tank and treated him with melafix but he only lasted 3 days. We hate to lose our fish and this has always been our easiest to maintain tank. Any suggestions would be appreciated. Thanks!

[lmw80]

Hi kayakers, and welcome to aquarium advice!

A couple things I don't see mentioneed in your post. First...the tests for ammonia, nitrate and nitrite....those are very critical conditions to keep track of. Second...how do you treat your water when you do a water change? Also, any signs of diseas, fungus, rot, etc. on the live or dead fish? Answering these questions will help us help you.

[Menagerie]

[center:d52f7f4fce] Welcome to AA, kayakers2 [/center:d52f7f4fce]

I see the ammonia level is undetectable; you should test for nitrate and nitrite. You can probably go to you LFS and ask them to test the water for you.
Just because you can't see signs off illness, doesn't mean there isn't one present. This illness may be in the gills and you would not see it. Have you added any new fish to this tank lately?
White fuzz could potentially be a couple of things. Look over these diagnostic charts and see if what your fish had was similar.

http://www.aquatronicsonline.com/hobbyist/hobbyist3.htm
http://www.clubjungle.com/fish_chart.pdf

BTW, what type of test kits are you using?

[kayakers2]

Hi again and thanks for the help. I just tested the water. I use an Aquarium Pharmaceuticals Master Test Kit. Ammonia is 0 ppm, PH is 7.0 ppm, Nitrite is 0 ppm GH is 35 ppm, KH is 35 ppm. When I do a water change I treat the water with Tetra Aquasafe as directed. I use a five gallon bucket and treat the bucket for five gallons. We have added some new fish and moved others over to the large tank. Over 6 weeks we added approx. 9 fish in 3 sets about 2 weeks apart. Looking at the charts the 1 fish that showed illness probably had a fungus all others appeared normal. Some of them had swollen abdomens but we have had a lot of babies appear. Could over feeding cause this problem? I do not have a way to test nitrate but I will take a sample to our LFS. Also I do add some salt... how much do mollies need?

[Menagerie]

Before you add salt--who else is in the tank? Generally, I keep 1 Tablespoon per 10 gals in my FW tanks. Mollies would like more, but I have not figured out how much they need. I have mine in what will be a BW tank, but I want to get everyone in there before I change from FW to BW.
Food can cause a swollen abdomen--either over feeding, or the fish may develop a blockage. If it is a blockage--try blanched peas (you will have to peel them). How often do you feed your fish?
I use those test kits also. I was wondering if you were using strips and getting "off" readings.

[Christmasfish]

I wonder if Aquasafe is one of those that messes up Nessler kits?
Just a thought...seems the more "bad stuff" a dechlorinator handles the more likely that it is not "test-friendly".