Poorly Variatus Platy

Not a full on emergency, but I am quite concerned about Daddy fish, my male v. platy.
The tank had it's usual clean and water change on Friday, and all seemed fine over the weekend.
Early evening he had his dinner and was out and about with the rest of the tank as usual, but later this evening I noticed he seemed quite subdued, so pulled up a chair and spent some time watching. He's not quite himself, seems to be hiding towards the floor of the tank and under the wood pile. When he is swimming he seems a bit wobbly - not full on swim bladder floating/sinking stuff, but not swimming completely upright, and ocassionaly wobbles and corrects himself.
I've spent the past hour testing the tank (0, 0, <40 with API kit, with pH 7.6 for the tank and 7.2 for tap water), doing a 20% water change, testing the tank again (same results), then testing again with a different kit (Colombo Aquatest) which gave 0, 0, 20. I also used it for some other tests, which gave the following KH 13, GH 19, PO4 5mg/l. No idea if this is having any impact - I know I live in a hard water area, but noticed the pH in my tanks getting higher over time, so have tested these parameters just in case, and so I can start to keep an eye on them. Will do another set of tests, including tap water, tomorrow. Water temperature is 20C.
I've also popped an airstone into the tank, mainly because it makes me feel a bit better if I think they've got plenty of oxygen when they are feeling poorly.
A bit tired now, so heading to bed, and will check on the little fella in the morning.
Any advice greatly welcome.
Thanks.

Sorry to read about this.

I know you do feed your fish a very varied diet but is there any possibility of being able to reduce the amount or proportion of dried food this week while increasing the proportion of live/frozen food or veg food, or even reducing feeding altogether this week? I have a lot of experience with swimbladder-type problems (from having kept goldfish) and this was always my first course of action before moving on to trying other options.

Update this afternoon, Daddy fish is swimming at the front of the tank with the white cloud mountain minnows. It's almost business as usual, but I haven't had enough time to observe him to check on his wobbly swimming.
I'm still going to go with the restricted diet advice, and I'll still check the water parameters and do water changes daily. Fingers crossed that everything is ok.

I meant to say, in my thread.....fingers crossed that your platy will be OK in the morning.

Daddy fish no longer hiding and seems to be fairly comfortable swimming around the front of the tank with the minnows. 
Will continue as planned and give an update towards the end of the weekend, but very hopeful that things are almost back to normal.
So, keeping in mind that I followed the dietary advice given by @fcmf my next questions are as follows:
have I been overfeeding him?
Was I not giving him a varied enough diet?
Should I consider more pea days?
What other veg is it safe to feed?
I don't want to see him miserable again, so want to make sure I'm doing the right thing for him and all temperate tankmates.
Also, thanks for your support & advice during this incident. 

If it was dwarf gourami iridovirus, it would have been infected before you bought it. There have been opinions expressed that all dwarf gouramis bred in the Far East are infected by the time they arrive in our shops. The only way to be sure a DG doesn't have the disease is to buy from someone who breeds them at home. You'll find a lot of experts (eg the people on WetWebMedia) who advise against buying dwarf gouramis just for this reason.

I know I won't be getting any more Dwarf Gouramies, again.

I set out to get a pair of Honey Gouramies, and due to my ignorance and Pets at Home, housing Dwarfs and Honeys together, I ended up getting 2 females of each species.

I keep calling the fish that died a "he", as that is what I thought I'd bought, and can't get out of the habit, lol.

It's something like 22% are infected when they leave the breeding farms but as it is so infectious, by the the time they've been transported, kept in wholesalers' tanks then shop tanks, the chances of any escaping the bug are almost zero.



Have I mentioned that P@H once tried to sell me a striped male DG and a powder blue male DG as a male and female? Even though I'd only just started keeping tropical fish and we didn't have broadband back then, even the books in the library told me - before going shopping - that I was given 2 males. He insisted that he'd given me a m/f pair when I queried him so I asked again at the checkout and the woman there so no, that's 2 males, go and get one changed. They didn't live very long; it may have been DG virus even though this was about 18 years ago, or it might have been my incompetence