Platy behavior/symptoms

[fishnub]

Hello All, 

Been a lurker for awhile, have a question so finally registered an account...

So, I have a male platy who has been hiding under some plants at the back bottom of the tank with fins clamped for the past four days. Not sure what to make of it. 

I did notice that the platy had a long stringy poop about five days ago, so my immediate thought is some kind of parasitic infection... but the platy has been in the tank for almost 8 months now, no new fish have been introduced in the past 5 months or so, and those fish went through quarantine with the recommend aquariumcoop med trio, and no other fish are showing similar symptoms. 

The tank they are currently in is approx. 64 gallons and fairly heavily planted and the hardscape layout is making netting the platy a real challenge (too many hiding spots, he can still move fine when he wants to it seems.)
Tank has 0 Ammonia, 0 Nitrite, ~20 Nitrate, ~40 KH (PPM), ~150 GH (PPM), ~7.4 pH, temp is ~76F

The only thing that I've changed about the tank recently is switching from easy-green to 2hr aquarist APT 3 liquid fertilizer.
Up until these recent symptoms started the platy was always a voracious eater, he has never appeared skinny and still doesn't. One thing that does come to mind is I left a large algae wafer in the tank overnight about a week ago. I usually remove the uneaten parts of the wafer after a few hours, I wouldn't rule out that the platy ate way more of it than they should have.

The platy was among the initial group of fish introduced into the tank, as such I did not quarantine them like I have with subsequent additions. Is it possible they had a latent parasitic infection that was dormant/not showing signs for 8 months? Can overfeeding on an algae wafers cause bowel obstructions? 

I finally seem to have gotten light and nutrients balanced and algae in check... I'm super apprehensive about wrecking the tank just to quarantine him and have him die regardless. That said, I have free time this weekend where I can try to get him out into quarantine. Before I act I'm curious about the community's thoughts and experience.

Thanks.

 

 

Edited March 22 by fishnub

[Colu]

Clamped fins can be a sign something off with your water parameters or disease your KH is to low for platys they like a KH of  80ppm - 160ppm I would add some crushed coral to gradually raise your KH you want to add 1 pound of crushed coral per 10 gallons low KH can cause a lot of health issues with liverbears @fishnub