hi guys! i am a total noob to filtration but i have a 3000 gal pond with a 4 kois that are about 5 inches long. now my question is, i am building a new filter and i heard bioballs are good but i cant afford them. so can lava rocks replace the bio balls? is there anything else i can use instead of bioballs that are reasonably cheaper?
this is the filter i will be building.
TIA!