Hello and welcome!
Sounds like you are looking at getting decent sized tanks for the fish.
Are you familiar with the nitrification cycle? The link in my signature has an article helpful information.
You can try more frequent water changes for mitrates, vacuuming the substrate especially under where you feed the most, reducing feeding amounts permanently or temperarily, split the feedings in 2 or 3 servings so it doesn't hit the tank all at one like with one feeding, rinse filter media with treated /dechlorinated water or use chemical means which might be the last option if the others don't help enough.
Nitrosorb is one I have used.