Her is a solution I worked out this afternoon to reduce water agitation and maintain filtration for my betta. It's a 20 gallon long with a Fluval C3 HOB. I bought a Fluval bio-foam sponge
(with one curved side and a straight side) and a couple suction cup heater holders. I took scissors and poked two holes in the straight side and pushed the heater holder prongs in as deep as they'd go and then stuck it to the back wall of the aquarium, under the water discharge. I took the other heater holder and stuck it under the foam and lifted as high above the water level as I could without compromising the placement (without putting too much pressure on the foam so it popped loose). The surface agitation is greatly reduced and the filter is actually more effective because there is an added component that gives a little extra filtration and works as an anti-ammonia treatment.