Atte gave you some good advice.
The brown diatoms are normal and will burn out over time.
Cut your feeding to once every 2 - 3 days and change to real food. Brine shrimp are considered to be like feeding them potato chips.
I use Rod's Food (frozen) but you can probably make do with some decent flake food like Ocean Nutrition Formula One, or any of a dozen others. I rotate between Rod's, Formula One, and freeze dried Mysis.
Are all the snails Gary? Like Hi, I'm Gary and this is my brother Gary, and this is my other brother, Gary???