It sounds like fungus to me, caused by reduced water quality from all of the fry. If nothing else get rid of the males so there will be no more breeding. Any traditional fungus cure will work in conjunction with frequent water changes. Please also remove the new carbon during treatment as it will remove the fungus treating chemicals from the water.

The platies and mollies enjoy water with a higher pH than the other fish also since in the wild they come from areas near the coast where salt water can mix. Mollies in fact can live in pure saltwater. If you have fish in the same tank that enjoy the same type of water things go better.

Lance

Read up on the dredded Columinar Bacteria. ... caused from the bacterium Chondrococcus columnaris. ... and an earlier post from me from long ago on this forum. *Although true fungus can set in after other forms, but usually FC)

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Dirty water sorry, (and your high nitrites nitrates and amnonia (usually travel together) mean same. You need to do water changes to bring that down asap. This stresses the fish and allows disease. FB is bacterial and goes by many names (fungus is usually on dead tissue eggs so on and with experience you can tell the difference.)

The tank is not stocked for best environment for the types of fish you have. I agree with above post. Mollies need some salt (please do research on all your fish types) and should be in a separate tank with proper ph, salt content and water hardness). They get sick fast without this as you've seen. Although the high n's will do any fish in fast!

Then neons need softer water and much larger school, smaller school means stress mean disease. Adding salt will stress them (unless very controlled in sick hospital tank, ie dribbled very slowly then back again after treatment) read up on neons please.

Fry need new homes baby tank meanwhile as tank is way overstocked. (forget that 1 inch rule - see how silly a ten inch pacu would be ok in ten gallon tank (NOT, lol).

It has more to do with stocking, filtrating and needs of fish.

Dianos need tons of swimming space and again a school.

Why not get more tanks or exchange some of your fish and have a species or two for that one tank.. again, neons and mollies platys not good together but def not the mollies just will always get sick without their salt (please research that to amount (they are not marine but more brackish) as it's late for me to do it). (but really can't because they are in a tank with sick fish). You should also always take out sick fish and treat them in hospital tank (lowered lights to reduce stress, heater, filter floss only if using meds) and to not stress rest of fish with treatment.

And, the pleco has to go, it gets huge and is dirty (poos a lot).

If you don't do the set up right, eventually all fish will get sick. FC is fast spreading and deadly. You can use salt for mollies as it is hard to culture fc in salt (I do once daily changes (dribble slowly) to help treat it... but this will surely do your neons in.

I would also advice to always do your own research on each species and not rely on sales staff. This is a great example of fish that should not be together. Best of luck Slm

http://forums.gardenweb.com/forums/load/aquarium/msg0122561828877.html

http://search.yahoo.com/search?ei=UTF-8&fr=slv8-tyc7&p=columnar+bacteria&SpellState=n-2274024398_q-w5UvcPkbrv4lu2Y73Akv9AAAAA%40%40&fr2=sp-top