I Can’t Keep My Platies Alive

I have no idea what I’m doing wrong. I’ve been buying platies one or two at a time for the last month or so. The most I had in the tank at one time was seven. They seem fine at first, but then loose interest in food and start hiding under the filter all day. Then they end up on the bottom breathing heavily for a couple days until they die. Those are the only symptoms. I’ve purchased them from multiple different establishments. I’ve only had two males, so it’s not that the females are dying from the stress of being chased. My tank water parameters look fine to me. The aquarium had been up and running for months before I started adding the platies. I even had some rainbow fish from a tank I was taking down at the time live in there for at least a month before I found homes for them.

Here is a quick list of tank specs and everything I’m using:

20 gallon tank size

Underwater filter with 40 gallon capacity

RO water

76 tank temp

Eco complete substrate

Dosing flourish fert in small amounts 2x a week

Lots of amazon swords for hiding

Below is a picture of the results from my test strips. It looks fine to me, but maybe there’s something wrong that I don’t realize. I also know that test strips may not be the most accurate way to test water and plan to take a sample into the LFS. This is really frustrating me to loose so many fish. I’ve spent so much time and money on this tank and it’s so disappointing to see it failing. I hope someone can tell me how to fix this.

 

Well the store will also use strips. The nitrite color looks wrong, likely it got ruined before that test. Could you try again, quickly dipping into the water, and without shaking excess water off, immediately lay it flat and wait 30 seconds. Then take a picture. Also, do you have an ammonia test strip? That’s arguably more important than all of those tests, not including nitrite. Is there anyway you could tell me the exact brand and type of filter that is? Or even more helpful, how many gallons per hour it filters?

 

Nitrate ju know

 

Are you remineralizing the ro water?

 

Sorry to hear about your troubles, I have a few questions if that's okay...
Was there always at least one fish in the tank?
Was there any point at which there were no fish at all, and if so, how long was it?
Is the nerite snail still alive?
Any chance your tap lines has been flushed or cleaned recently?

 

Are you remineralizing the ro water?

according to her test, the GH, KH, and PH are all moderate, so she has to be remineralizing or her RO doesn’t work well with removing dissolved solids.

Sorry to hear about your troubles, I have a few questions if that's okay...
Was there always at least one fish in the tank?
Was there any point at which there were no fish at all, and if so, how long was it?
Is the nerite snail still alive?
Any chance your tap lines has been flushed or cleaned recently?

Shes using RO, so the tap line flush wouldn’t be a concern. I’m also interested in the other questions you asked. Guess we’ll see!