I think you might need more light actually. I'm in Florida and have a pint-sized no-tech bowl in a southern window that gets sun off and on all day and it's doing really well. Sand bottom, pennywort, oscmocote granules in the sand. I have to rub off a clear slime coat on rim each week but otherwise it's as clear as the day I set it up. I do a 100% water change each week just because it's so easy. Pics are in the low tech forum.
Years ago I also had a 29g in a western window that did wonderfully with hot blazing afternoon sun.