Hi and welcome to FnF. I am sorry it is under these circumstances.

Your pics didn't post in your other thread. Here is a link for posting pics.

Can you fill out these questions to the best of your abilities. If you don't know, that is fine, just fill out what you can.

I can tell that you have done your homework but there are a few things that jump out at me. Do you know if your tank was cycled while the fish were in it or did you use something from an established tank or BioSpira? Spikes in ammonia and nitrites are toxic, especially when fish are new. Illness or spikes in ammonia and/or nitrites were likely the cause of the initial losses.

Next, distilled water doesn't have the minerals that a tank needs. A difference in pH between the distilled water and your tap water may have further stressed them out. I would go back to using your regular tap water and treating it with a water conditioner.

Livebearers, the mollies and platies, do best when there is 2 females per male. Males tend to be to chase a lone female around nonstop and stress them out and this can lead to disease or death.

Adding fish without quarantining them in a quarantine tank can introduce new disease or illness to a tank.

Did the pleco actually grow from 2 inches to 7-8 inches in 3 months? On the questionaire linked above, please fill out the amount, frequency, and type of food you are giving.

Ich looks like little grains of salt or sugar. There are safer meds available for ich.

There is no need to replace the ceramic filter media (BioMax.) This is where the good guy bacteria calls home and when replaced, spikes in ammonia or nitrites can occur.

Activated carbon can be added once the recommended doses of medications are done but is not needed in healthy tanks. Some of the bacteria will also take hold on the carbon. So by removing both the carbon and the BioMax, your tank probably had some ammonia and/or nitrite spikes. The nitrate reducer is not needed. In my tanks I have a mix of BioMax and filter sponges and neither have been replaced but simply swished in old tank water when they get too gunky.

If you can please fill out the questionaire and also try and get pics of the sick platy, that will help the ER mods give you a plan of attack. Hang in there, we can help you through this and you will have a healthy tank that your whole family can enjoy!

-Shelly

* edited to fix ER questionnaire link