Andy, I reckon a ND filter would be the way to go. I've recently bought a ND grad filter.
this is only a guess, but are you now taking photos using MH lighting, and they now seem 'fuzzy'?...more than before?
light spill from MH lighting is much more than luminaries. for one, their higher so light spill is much more.
the closer to the tank the more 'spill' you'll get. It's a like a triangle of light. try taping a piece of cardboard the light fitting and let it hang down.