Yes I took marine science for four years way back when. Kind of convenient when your school is about five miles from the ocean xD I had a simply ridiculous amount of science credits by the time I graduated, as I opted to take science classes for extra-curricular classes (nerd? me? oh yes, I'm afraid so). That was back when they also first offered marine science as an AP course (course you can get college credit for when still in highschool) we were one of the first test groups, tiny class, there were only like... 16 of us? My final project was successfully breeding and raising over fifty seahorses.
Yeah, total topic derailment right there *drags self back on course*
I remember we used to make filters out of soda bottles, but I'll need to draw up a diagram (or see if someone else has one floating around online, honeslty I'm sure my class can't have been the only one to have ever used this method) because trying to explain it in text alone is just annoying and confusing. Tomorrow I will. But they are cheap, and about the only thing of any cost you really need to buy is a pump for said filter.
Pumps... hmm. When we were rigging up several of the tank branches in our classroom we managed to get our hands on several pump that used to be used by our local county water department that they were throwing out as 'junk' because they could no longer run at the high PSI they demanded without the seals leaking (it was something ridiculous like over 200 PSI) but they were still perfect (and didn't leak) at a more normal PSI, and we were able to rig up several different pumps for different tanks off of thsoe. I doubt you'd be able to get ahold of such pumps though, it was a pretty odd event. Eh, that's not really helpful, would probably be cheaper and more efficient to the size of tanks you are working with maybe to just get the pumps from your local store.