Haytrev, I posted this on your other thread, so will be brief. The fish you already have are already overstocked, they must be babies at this point, but we always count them as adult size for stocking. Both the columbian shark and mono angels get around 14", and have to be counted in 1 cubic inch per gallon.
The nitrate problems you have experienced, and algae are both a direct result of this, and soon you need to be looking at getting a bigger tank. For the short term though you need more filtration. The Fluval 205 is only rated for 40 gallons of tank, you have a 50 gal. It processes 110 gph, which also isn't enough, though you don't count a canister filter the same as a HOB filter.
I suggest you add a aquaclear 110 to your current filtration to provide the water movement and good filtration your fish require, and not add any more fish until you get a bigger tank.
