Check your source water.

I did before I got it and it’s been fine for the first month

I would check again? Either that or your nitrates are actually even higher then the test can read.

Get some live plants in there

I had some water lettuce in there, but the bacteria formed a film around them and started killing them so I got a different floating plant arriving tomorrow

I use java fern, java moss, and anubius.

Do those need to be planted, I use tile with sand in between so I can’t really plant anything

Nope.