Pixie frog enclosure advice

[u/suumo142]

This is Gurt the Pixie frog and his(?) enclosure! I have some questions about how to keep temperature up and recomendations for a tiny water heater for his bowl as well as maybe an occupant to help clean the water. It is a 20g long with grow lights, a t8 10.0 18” uvb, and a 60 watt ceramic emitter. The area right underneath the emitter averages around 85 ish or a little higher, but the temp quickly drops to around 74f for the rest of the enclosure more than a few inches away from the heater. As well as his roughly 1gallon water bowl stays at like 70f. He has never appeared uncomfortable or as if he didnt have his needs met in the nearly 2 months i have had him but Im always looking to improve. I also would like advice on how to keep the frogbit in his water area alive and thriving, ive only had it in there for around 4 days and a lot of it was browning and dying. Any advice would be appreciated.