I am wondering if the T5 lighting is not too much ,too warm.
I grow floating pennywort in 55 gal livebearer tank under dual T8 bulbs and Aquaclear110 along with two HYDRO V sponge filter's that run 24/7, so I do not believe the sponge filter makes a difference.
Might consider Seachem's flourish comprehensive as a source of nutrient's albeit more of a trace mineral compared to the macronutrient content.
Is better than no nutrient's at all, especially if fishes or invert's are not contributing,and even more so, if CO2 is being used. with CO2 you will likely need full range of nutrient's
Maybe try first raising light fixture,shielding light with window screen.limiting lighting to eight hours if this is low tech. This would create less demand for nutrient's and the flourish comprehensive could help.