Fish flicking, erratic swimming, going crazy

Hi, I’m still quite new to the hobby and I’ve been having a few problems since I upgraded my tank I think about 6 weeks or more ago.
I moved things across and this tank is bigger.

I just did a water change and my platy is going crazy. Flicking, clamped fins, swimming erratically. I don’t know what to do?
I changed around about 60% of the water as I have another platy that is just laying on the gravel and seems to be struggling to swim.
I can’t see anything on them.

im really concerned.

pictures/ video?
ammonia, nitrite, nitrate, ph?
did you dechlorinate the new water before adding it to the tank?
do you have any aeration in the tank?

I don’t know what they are as I have taken my water to the fish shop to get it tested.
I used api stress coat.
I have two filters that create bubbles and a ripple on the surface.
One fish has been laying on the bottom for a few days, he’s not got anything on his body. He swam up when I changed the water and seemed to go upside down for a moment. He’s clamped and this has been since the water change. Flicking like crazy.
Should I add a medicine?

the other: his colour has drained, swimming erratically,

no you don't add chemicals unless you know what the problem is.

take a video of the fish and upload it to YouTube then copy & paste the link here.
and get some better pictures where the fish aren't facing the camera or hiding in a corner.

get the tank water tested when you can and write the results down in numbers.

The fact that you're using a water conditioner for water changes (API Stress Coat) indicates that it isnt a chlorine issue.

It would be helpful to know some parameters of your water, both tap and tank.

You can go to your water supplier's website and grab a water report (you input your postcode) and that will tell you things like water hardness (GH) and alkalinity (KH) and also PH.

KH and PH are quite important because if they are low, then your PH may have dropped in-between water changes which may be the cause of the problems you're having now.

Otherwise yes, as Colin says, for immediate diagnosis of your issue, we'd need to know Ammonia, Nitrite and Nitrate measurements.