Hi Happy Holly,
This is a little late, sorry, only just joined :-)
I to had this problem, had a MASSIVE tank, with about 20 Tuna, and it was always filthy.
No matter how big your tank is, you have to much fish, which is why the tank got dirty.
Reduce to about 6 fish max, then your water will always stay clear. Learnt this the hard way :-) Looks a little odd, having this enormous tank with only 6 fish, but happiness is almost always 100 and water is always clear !
Lee