Oh No! What's Wrong?

[AquaKai]

My male crowntail betta is acting really weird. I've had him about a month now. He is sort of "sitting" vertically and breathing heavily. Occasionally he swims to sometihing but always comes back. This has been happening for the last few hours. Is there something wrong?
The parameters:
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: >5ppm
Temp: 79 degrees

I just did a PWC today, I did the dechlorinator like always. I use Seachem Prime. What's wrong?

Pictures:

[Oldfishlady]

Did it start right after the water change....

How much and how often are the water changes, water temp.....

I would make another water change and dose the tank for the full volume of the tank and make sure the replacement water is close in temp as the tank water..

[AquaKai]

It happened a little before the water change that's why I did one that was off schedual. I usually do a 20%-25% PWC every Saturday. The pH was the same, 7.2, as was the temperature. I'll do another PWC as soon as I can. Thanks!

Oh and I forgot these the last post:
it's 8g and has a tetra whisper 10 filter. The tank has been up since mid June and I haven't had any issues thus far.