Platies and mollies, problems with mollies

Question
We have a 55 or 75 gallon tank (inherited tank with house).  It has been set up for at least a year, don't know how long the previous owners had it going.  There is also an apple snail, a pleco, and four ghost shrimp.  But, it is mostly stocked with mollies and platies.  Lately, the mollies have been dying, about one per week.  The afflicted fish will have a droop to their tail and have trouble getting around.  They do seem to continue to eat, though.  Othwise, they don't have other symptoms.  One will die and in a couple days another will be labored like the previous.

Our nitrites have read particularly high lately, pH is 7.2 roughly.  Ammonia and nitrates are a bit elevated, but the nitrites are almost off the chart.  We added Nitra-Zorb and Bio Chem-Zorb to our filter.  Nitrites went down for a couple days, then went way up again.  We do about a 20-25% water change every week or so, but we are trying to do more like twice a week now.  The platies don't seem to be affected.  We did have a case of bloat a month or two ago that claimed the lives of a couple fish.  They had whitish stringy poop and some of the sick fish have that now as well, but they do not look bloated.

Any ideas how we can stop this or what it is?

Answer
Hi Neal,

Since there are no obvious characteristic symptoms of disease, the most likely cause would be ammonia/nitrite poisoning.

The most likely cause is that somehow, the beneficial bacteira which converts toxic substances (ammonia, nitrite), were destroyed. There's nothing you can do to restore these bacteria, they will take a month to grow back.

In the meantime, perform 20% water changes with a gravel vacuuming every DAY, for one week. This will help dilute the ammonia and nitrite in your tank. ANY levels of ammonia and nitrite are dangerous to fish, the only safe reading is zero.

Bloat (dropsy) is hard to cure, but since it's normally caused by bacteria, you may find that treating it with any anti-bacterial medication, such as Maracyn, will help the fish greatly. I'd so it as a precautionary measure.

Good Luck, and I hope your fish recover!