If the 300 has slots in the back for HOBs... I would get 2 used AC110's and 1 used FX5 or 405 if your budget is $250. 2 used ACs and 1 405 will hold more media and use less electricity than 1 FX5. I currently run 2 FX5 on my tank but had to remove 2 trays from one of my FX5 to make it hold more bio-rings.