This morning I found one of my male cherry barbs sideways and breathing really hard. The other fish look and act normal, super active and still eating 

Parameters are:

Ph: 8.0

Ammonia: 0ppm

Nitrite: 0ppm

Nitrate: 0ppm

Kh: 12 dkh

Gh: 5dgh 

The tank is a year old, 20 gallons, well planted,  9 cherry barbs plus 5-10 fry. I do water changes every other week, I feed them four to five days in a row then don’t feed over the weekends (I’m not home). 

I’m not sure if this has anything to do with this but I haven’t been able to raise the nitrates since I cycled the tank, I’ve tried a little over feeding, extra fertilizer but I just ended up with more snails and algae.

The water level is low from evaporation and also my filter will get upset if I have it too high 

The adult barbs are about 2 years old now, the male in the video looked just fine yesterday before the lights turned off. I’m somewhat surprised he’s still colored up, I would assume a fish like this would be washed out from stress or illness? Besides being sideways and the labored breathing he looks healthy, no spots or injuries or anything 

If anyone could help me figure out what’s wrong with this guy or what to do, thank you for your advice!

What could be going on with this guy?