New platy hangs out in top corners

We have had our first fish (3 mickey mouse platies and 3 swordtails, all six are males) in our cycled 20g tank for about a week now. For the last three days, one of the platies hangs out in the top corners of the tank (rotates corners periodically) and doesn't eat much, if anything at all. The other 5 fish are doing great, swimming all over, interacting with each other, eating plenty.

I test the water daily and it is green across the board so no apparent water quality issues (plus the other five seem great). I'm a beginner but it doesn't seem to be something like ich.

Any thoughts? Not sure if it will work but here's a pic of our corner guy. Thanks

Have his gills always looked that red?
Can you give us your exact water parameters, including KH and GH?

Thanks for the reply. Not really sure about his gills because not something I noticed. Here's a pic of the other two seemingly healthy platies. I don't have the exact numbers because I'm using the 6 in 1 test strips.

Could you post pictures please of each strip result against the grading as the guys here will be able to help with the results

I'd a new Molly do this while she was settling in. There was an aggressive fish in the tank and she just felt safer there. But I would get an Ammonia test. The strips don't test Ammonia and it's a really important number to keep an eye on. The API Master Test Kit is really good, seems pricey but you get about 800 tests out of it so worth it.
(Edit typo. That should actually just me my signature)

Thanks.

We need to see the strip next to the chart on the bottle so we know what each test is, and what the colours are for each level.

The alkalinity goes back and forth it seems every test I do. I also tested the ammonia today and it was "ideal" so not thinking it is a water quality issue. Maybe just a weak/sick fish. He's hanging out in the bottom corner today, not swimming all that well.

I would be wary of what the instruction say are 'ideal' or whatever.
The only 'ideal' for ammonia and nitrite is zero.
Nitrate should be under 20.
There is no 'ideal' for hardness (GH), alkalinity (KH) or pH. The levels for those are what comes out of the tap. GH and KH could be anywhere between zero and 25 dH; pH could be anywhere from below 6 to above 8. And different fish need different GH and pH, for example platies need hard water and high pH.



What are the numbers for GH and KH? Not a photo of the strip but the numbers from the chart.

So for GH somewhere around the 150 color (hard) and for KH around the 120 color.

Took another snap of him today, not sure if this black spot between his eye and gil is a concern. Don't think it is ich.

So I've now done multiple water tests (6 in 1 and ammonia) and they all come back good so the fish itself just be the problem. I've also done a 25-30% water change. I'm actually amazed the little creature is still alive at this point - hasn't eaten in at least 4-5 days.