what to do?

my 120 reef is going really well and it's at the point where other than routine maintenance and adding corals theres nothing for me two do but enjoy it,, as happy as that makes me i'm jonesing to build something,anything! so here we go...
this is a project i've been working on for a while now, i had so much old equipment/tanks etc kicking around that i wanted to use and wasn't sure how, the only rule i had starting this was that i could only spend $10 on it. i've spent 3 so far so that leaves a $7 budget!lol

this tank is going to house native new england creatures since i live so close to the beach, the top tank is a tidepool for intertidal plants and snails it's a 20 long,the substrate is an absolutely filthy mix of local river mud,topsoil from my backyard,crushed coral,beachsand{from the ocean not the store}and aquarium rocks from the store,basically all the substrates from all the tanks i recently broke down,it goes up and out of the water in spots ,the middle is a 29 and will house stars,urchins,fish,crabs,shrimp and anything else i can catch at the beach! the 29 is jammed to the gills with homerade live rock and rocks left over from my african cichlid days{sorry the pic sucks},
i built the stand at work for free{its ugly but it's in my "romper room" where anything goes so it's ok}the top tank is drilled to fit a $3 sink bulkhead from home depot and drains into the 29 at roughly 100 gallons an hour, an old fluval canister is the pump, a cool unexpected thing is that the drain is a little small for the turnover it sees and surges every 3 seconds or so when the fluval is opened all the way up,i can trim the flow down for feeding or when the noise gets on my nerves.the 29 has a strong airpump and airstone in it,there's 40 watts of 6500k up top and 128 in the middle.not alot but thats what i have to work with.
i may change that around when i see what will grow and where,
the question is what do i do with the third{20 long} tank? i could plumb it in but to what purpose? or leave it unplumbed and i could try my luck at breeding brine shrimp down there? or plumb it in and keep it for larger stuff?maybe even a pet lobster?
i still have a boatload of extra equipment kicking around{no more extra lighting though} and the airpump could run a few more things as well,
i want to do something down there but am short on ideas,
any input/ideas are greatly appreciated...

 

Sounds like you've got quite the setup going there... 3 dollars so far.. nice I don't deal much with fresh water so i have no clue. but it seems this is going to be a cool setup once done.

 

Sweet....

 

saltwater baby! i don't think i'll ever go back to fresh,so far folks have suggested an algal filter and a cryptic zone, i'm leaning towards pet lobster still myself

 

oh. i assumed by substrate from your back yard this was gonna be freshie tank.. my mistake ><