What do you use to keep your water crystal clear besides W/C's? Can't really run two filters because that'll be to much flow for my bettas. When I had my old 29g years back the water was always nice and clear because of my canister filter now I can't seem to get water that clear with a HOB filter.
Maybe your under filtered with the one HOB?
What size tank? what HOB ?
Not many risk keeping more than one betta per tank unless the tank is larger, and feeding one or two small betta's should not pose problem with water clarity unless they are being grossly overfed and or as mentioned the tank is under filtered or overstocked.
Betta's in my view are possibly the most abused fish in the hobby and I have heard all kind's of nonsense .."My betta can't have any flow or he/she pout's" or "Betta's don't like large tanks cause they're used to bein in puddles".
"Betta's don't need filter's or heater's so they are easier to keep than most fish".
The fish can easily handle moderate flow from filter's so long as there is a area of lesser flow that they can retire to and or the flow is not so much as to cause the fin's to become ripped up.
They need same thing's that all tropical's need Filtered tank,heater,good maint.
My two cent's