To attack the brown algae/diatoms directly I've used chinese algae eaters in my african tanks but you might have to search for one already grown a little or raise one for a while seperately until it's large enough not to be eaten by the africans.
You can probably get by without the buffers which should help also, especially if they're loaded with phosphate which some brands are.
Foods with higher digestibility should allow you to feed smaller portions having less contribute to the nitrate coming from the fish and better quality flakes that aren't double cooked should provide less powdery mess that dissolves and can't be eaten by anyone. My first choices for that goal would be Omega's African mix and their Super Veggie Flake by Omega availible at Petsmart or the Ocean Nutrition line which Omega manufactures for them.
http://www.omegasea.net/
If the fish are grown and not actively raising young/breeding, once a day is plenty, twice if you keep the portions small and added slowly so all of it gets consummed.
I also started experimenting with Seachems Matrix added to my filters but had to move prior to getting solid results with data. It's basically a rocky/gravely looking addition that has micropores. The micropores allow bacteria to develop all over the media and into the outter sections of the pores serving as standard biological filtration converting ammonia out to nitrate then in the inner pores anaerobic bacteria develop and finish converting the nitrate back out of the water completely. In other words total biological filtration. The Penguin 350 could have it added into the baskets, the Fluval could probably have some added also. I think I saw it at Petsmart also.
