Fish deaths

Question
Hi, I am pretty new the fish-keeping business. I have a cycled 30g tank setup with 4 balloon mollies (was 6,) 2 danios, 3 corydoras, and an outbreak of snails. Yesterday morning I woke up to find my male mollie floating at the top. Thinking he may have just died of natural causes, I was upset to find one of my female mollies dead under a plant, obviously having been there a couple of days and munched on by the corys. I took the lid off and scrubbed off some green algae with hot salt water, and then did a 45% water change and gravel vacuum. However, I am very worried about my other fish. Water parameters tested normal, my tap water is hard but I don't think that is a problem. Heater is functional, but used, and the temp feels a little different every time I touch the water. I thought this was just a function of it turning itself off when the desired temperature is stable, and isn't an extreme change. I have yet to change the filter media, as it has only been a couple of months since I set up the tank, and I was afraid of hurting the cycling process. Now one of my smallest females is sucking bubbles at the top and sliding her sides and belly along the glass and other things in the tank, and just overall looks kind of droopy. I know this quickly points to skin parasites, but I didn't see any parasites on the males body, and never noticed him rubbing on things. He was acting a little out of it, not chasing the ladies and kind of chilling in the top corner of the tank before he died. I do have a couple of bottles with malachite green and formalin, but have been hesitant to use these because I'm not sure if that's the problem. I don't really know where to go from here, I want to treat as soon as possible if it is a parasite, but don't want to be hasty to put chemicals in my water, especially with catfish. Thank you for any suggestions you may have!

Answer
Hi Allie,
Sorry to hear that...
I know you said you tested your water. Did you make sure the ammonia and nitrites were definitely ZERO? And that the nitrates weren't too high (around 20 or less is best)

I know you just did a water change too but it wouldn't hurt to test your water again since the one molly is acting so droopy and sickly. I'm not too sure about immediately blaming parasites for the symptoms. It doesn't always mean parasites. Does it look like she is shimmying?

Ideally I'd isolate her in a separate tank. But if that's not possible you just may be stuck with the main aquarium. Typical advice for symptoms like this is a water change, (which you've already done) and aquarium salt treatment. Now Cories often do not tolerate salt very well. But lots of people use a small dosage short-term and the cories do fine.

This is the best I can recommend so far for your sickly mollies. More water changes and a mild salt treatment (slowly add the dose at intervals after pre-dissolving the salt in a cup of water) I hate to advise medications now because we aren't sure what we are treating for exactly.

Mollies tend to be more sensitive to water quality than other fish. Sometimes they thrive best with some salt in their water but then again they can do very well with freshwater but it must be kept very clean and a reasonable consistent temperature of around 78-80F.

Best of luck and I really hope all goes well!
Karen~