Fish Acting Very Strange! Help!

[-FighterFishh]

Hello all!
For about the last 2 weeks, my male swordtail in my 64L has been acting very strange (stocking in my sig). He's been hiding under a piece of slate and breathing very fast, not even venturing out to eat. For the last few days, I've also noticed my 3 hybrids and my female swordtail have also become very skittish, jumping at the slightest sound and swimming away when someone approaches the tank, and also occasionally flicking however they are still eat normally and don't have any outward symptoms (no white spots or anything). The endlers, corydoras and swordtail babies are acting normal, though, coming up to feed and swimming around as usual.

Water stats are at...
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 15ppm

My test kit is API liquid test kit.
Tank is fully cycled and has been running for over a year now. It's fully planted with lots of cabomba, 2 pieces of bogwood and a few slate caves.

Poo seems normal and everyone is eating except for my male swordtail. I finished a course of eSHa 2000 today which I put in to try and see if it helped with the male sword, but it hasn't made a change. I'm planning to do a large w/c tomorrow and Sunday to see if that improves things, but would seriously appreciate any help anyone can give!

Any advice is much appreciated, thank you!

 

[Cazgar]

You should be showing some nitrates if your filter is fully cycled, give the bottle a good shake & test again.
A large water change is a good idea, have you added any new fish or anything new in the tank recently?

 

[-FighterFishh]

You should be showing some nitrates if your filter is fully cycled, give the bottle a good shake & test again.
A large water change is a good idea, have you added any new fish or anything new in the tank recently?

I find this tank doesn't ever show any level of nitrates, but I just put it down to it being heavily planted? I'll check again for you in a sec

Only recent additions are the cories about 4 weeks ago, and they're acting normally. Will do the big w/c as soon as I wake up tomorrow morning, hopefully that'll improve his condition a bit. Do you think it'd be worth trap netting him so he eats or should I leave him where he is?

Edit: Also, just remembered my hybrids and my female sword occasionally flick on the sand or ornaments but have no outward sign of infection. Added the nitrite levels to the main post, shaking the bottle seemed to do the trick!