Do larger waer changes or purcahse a larger filter Eheim 2260 or 2262 would do wonders on that tank. I do not see where a UV sterlizer would benefit you.
You're lacking bio media. Just go to your lfs and get as much bio media as you can cram in your canisters. Trash the carbon, and replace it w rings. Do this first before anything and give it a few days for bacteria to grow. An eheim 2260 along w what you have is not needed. They're great filters, but you need bio media
You didn't mention canister maintenance but if your cleaning them all the time that will kill some bb. Regardless if you have only one tray of bio, you're still lacking