On my fixed gear my chain seems to tighten and loosen as I pedal, im thinking its the sprocket or something. Could someone help on the subject?

Chainrings are not often perfectly round, which can cause the chain tension to vary. Sometimes it just needs to be centered on the spider, sometimes it just sucks.

https://sheldonbrown.com/fixed-conversion.html#tension

If there is a little bit of play where the chainring bolts onto the cranks you can probably recentre it. Loosen the bolts a turn or two, and try wiggling the chainring around.

You can try the Sheldon Brown method; I personally got nowhere with it after an hour of fiddling. I'd just loosen all five bolts with the chain off and get it as centered as possible. Most drivetrains won't be perfect (chainline, centered chainring, etc.) so just do the best you can.

When you've got as far as you can with the adjustments try to tension your chain while the cranks are in the tightest (chain tension-wise) position. If you tension at the loose point you risk having the chain bind up when it gets to the tight spot.

Edit: 1000th post!!!!

If you tighten your chain to the correct tension, even if you do so at the loosest point, it should not be anywhere near binding on the tight spot. If it is you are likely using a bio-pace ring. Seriously no chainring should be that out of round. I can move my chain almost a half inch up and down with my fingers, which means it has about 7/8ths inches of play. if your chain ring is almost an inch out of round, buy a new one please

Hey man, biopace works fine with FG

dad used to service manufacturing machines that used what were essentially large bike chains:

"as long as you can keep play less than 1.5x the width of the chain at the loosest point, don't worry about it."

fwiw....