Magnum 350 with overflow help?

Hey guys, this is my first post but i have been looking around on here for ages and first of all thanks for all the help and info on cichlids and freshwater tanks in general, this is a priceless site. k, onward to the questions. I have a 75 Gallon with a corner overflow, and a magnum 350 for the filteration. When i bought the tank the pvc was cut down to the botom with a right angle and a hole drilled at the top, so i got some lava rock, broke it into some various sized pieces (nothing bigger than 2x2") then through an airstone under it to provide air to the bacteria. Overall, it seems to be workin pretty good, but the water isn't crystal clear, like i have had with my previous tanks. My question being, what filter should i use inside the magnum? the micron? the rite-size over priced blue quilt batting? or fill the carbon in the media container, or put more small lava rocks in? etc. Also, i have a pond pump laying around that moves 258GPH, and i was wondering where that could best be used, could i put it after the canister to provide more water movement? Should i make a new powerhead? (I only hve a 170GPH as it is now in that department)? Sorry about all the questions and thanks for your replies.

Fish wise:
7 inch dragon Catfish
4" fahaka puffer (i know he's supposed to be mean, but mine has been cool the entire time i've had him and hasn't touched any other fish...bit me once tho....)
2 clown loaches
~20 various cichlids
rubber lip pleco
some anacharis, italian val, and other plants