Cannister filters and no-filters...are for different kinds of tanks: a simple guide

[Sk8r]

If you have a fish-only, a cannister can work. We used to make them work when it was a plastic pot with carbon and floss. A modern multimedia cannister can do a pretty good job for fish only, if you keep it clean. Track your alkalinity obsessively. Water changes and buffer addition and filter cleaning will keep you in good shape.

If you want to edge over into corals, a cannister will still support a buttons-and-mushrooms-green-star polyp tank pretty well. All corals are living filters, and the hardier varieties will likely do all right. I'd stick to hard discosoma shrooms rather than ricordeas, however. And use a reef salt instead of the cheaper fish-only salt: the difference is in the amount of calcium. Track alkalinity closely. Water changes and buffer addition. And clean the filter.

If you want a reef for sure, time to add a downflow box and the plumbing for that downflow to feed a sump, with a skimmer and return pump, but either no filter or a simple filter sock to catch the floaty bits. Again, soft corals and even lps are perfectly happy to clean your water for you, so floaty bits are ok.
You DO need 2 lbs of LIVE ROCK per gallon of this sort of tank. This rock [and the aragonite mineral sand that goes with it] IS your filter: its bacteria process the waste from fish, its skimmer!!!! sucks out the spare amino acids and purifies the water---what the surf does for the ocean---; and in general it fares far better without a filter at all. The fussier (and spendier) softies do well with this, as do LPS (large fluffy corals with stony skeleton). For LPS you do have to add calcium---a lot of it. For stony, track (in addition to alkalinity) calcium and magnesium levels.

[Tarawa]

that live rock, can it be dead rock, which would eventually become live?

[Sk8r]

Yes. The life, however, ---variable. If you cycle with a rock ONLY having bacteria, all your rock has only bacteria---takes 8-12 weeks to really get the rock toward live (the bacteria needs to reproduce a lot, and needs to work inward past the 'skin' of the rock.

There is also, same term, live rock which comes in not only with bacteria but with worms and copepods and all sorts of good things which will multiply (given enough food) to swarm over the rocks. Ultimately you want that to happen, because a 'really' live rock base processes fish waste really well and provides a lot of benefit to the tank. [Those keeping sps corals, the colored sticks, want to go the other direction and limit how much stuff is crawling about---and keep the water hyper-clean.] Those keeping lps and softie corals go the other way---has to do with whether, in the wild, the coral develops best in certain areas of the reef.] But if you only have access to bacteria-laden rock, that will do nicely, and sooner or later, the worms will come. Or you can order them from some speciality companies. Anyway, to get the rock 'fed', you can just drop in 4 flakes of fish food for each 50 gallons of water daily, and you'll keep your bacteria happy and be on the way to cycle. Magic potions and dead shrimp and so on don't do it any faster. Bacteria reproduce about the same rate no matter what you do as long as they're at 80 degree temperature in good salt water and have a steady food source.

If you have a fish-only tank, and want to build up your rock supply, just put in a small dry rock now and again and pretty steadily run a gfo reactor to take the phosphate out (comes in with the rock)---the rock will algae-up, the fish will eat the algae (assuming you have at least one that will) and the rock will absorb bacteria and turn brown. Once that rock has processed, add another rock, etc. As long as the gfo is removing the phosphate the rock comes in with, and you don't push it too fast, you can slowly build a rocky reef out of rock that's becoming live rock. Which helps process fish waste. I do this in my regular lps reef. I keep a store of spare rock in my sump, just there if I decide I want to build an arch or throw up a rock spire to confuse two fish who are getting rowdy along a territorial border.