What is wrong with my platy?

So, my fish tank is a 180-185 litre, I can't remember and in it I have:

3 Neon Tetras

3 Glowlight tetras

2 Red phantom tetras

5 Pearl danios

3 harlequin Rasboras

1 bn pleco

2 male platies.

Here are the problems:

My yellow platy is on the bottom of my rock formation just sitting there, with it's large belly on the bottom of the rock formation. He is flicking his fins and it looks as if he cen't swim and is really struggling.

My other platy, a sailfin platy who normally has a really nice,long fin on the top has that fin down and tight to his body. My water paramaters are as follows:

Ph.7.2

Ammonia:0

Nitrite: 0

Nitrate:40

I know that the nitrate is a bit high, I have done daily water changes to try and cure this and it is not going down, is that the problem.

The fish is not getting bullied and he is probably the favourite fish in my eyes. I really don't want to lose him.

Please help me!!