Damping off is the simple name for pythium rot. It usually shows in the seedling stage, the seedlings' stems darken, get thinner, and eventually they just fall over and die.

I also recommend silica blast. Strengthens cells against heat and insect damage, and contains minerals that HELP BUFFER YOUR PH!!! YAY!

BTW I'm stupid and can't figure out if it's a soilless/hydro setup or automatically-irrigated soil.