Did you slow down the filter at all? My betta seemingly just hides under the stem plants and does not much swimming anymore.
When I had the HOB in, I didn't slow down the flow. My Zoo Med doesn't put out enough flow to bother the Betta though, he doesn't mind being any place in tank with just the Zoo Med 501, he's just on the lazier side. Temp is about 73-74 which is on the low side, reason I am thinking a heater. I have been told by a few people that I need to be about 72-73 before a heater should become a concern, but he will slow down. The last tank he was in was about 80 degrees and he was active.
I am going to try a Fluval 25 watt as it is pretty unassuming and made for a 6 gallon tank. It's not adjustable which doesn't bug me it itself as I would have choose the same temp. However, the only non adjustable heaters I have had (all Tetra brand) have failed on me. It sucks because I have 4 heaters but they are too big, both size wise and wattage wise. I think only one will fit in the tank at all.