The other thing to check is to make sure it's cyano and not dinoflagellates. If your corals are dying I would suspect dino's (aka brown snot algae)

I used to run a cannister when I first started as well. Who has'nt? If you do plan on running your tank with what you have than I recommend you do this. Take everything out of the cannister and just use live rock rubble instead. You can also put a bag of carbon in as well. Just like others have said, "it's just a nitrate trap."

+10 on getting rid of the fluval, when I started out I was running a fluval 404 and after about a year I started having big algae problems, ditched the filter added a few mexican turbo snails and things turned around completely.

Ditch the Koralia powerheads. They are known for their seal letting up and allowing water into the motor housing causing stray voltage to run through your tank. Best test is to find a cut on your hands, and while barefoot, touch the water. If you get zapped, you have stray voltage. If not, then you are ok.

The cyano issue is always very simple, there is too many nutrients in your water. The water from your LFS is probably high in total disolved solids (TDS). Get yourself a RO/DI unit. Trust me, it will give you sanity.

Vlad.

WHAT??? what are you talking about???

where did you get that? I never heard of koralia pump causing stray voltage. I have 2 in my tanks so I want to read about that for sure.

I am sure there are better way to check for stray voltage than get zapped!

thats cuz its an old, old issue from when these first came out.

one that was addressed by Hydor years ago.

I've had 3 Koralia's in my tank for a couple years now with now issues

consider a remote deep sand bed. a rubbermaid office garbage container or a salt bucket will do, fill with 8 inches of sugarfine reef sand, flow about 30 gph water across the top of the sand with the water about 2 in above the sand, keep it higher than your sump so you can gravity feed the water back to your sump, keep it covered and in the dark, it will take about 2 weeks for it to kick in, will reduce nitrates, i added one to my 90gal heavily stocked mixed reef and dropped my nitrates from 10 to 1, be patient and keep up the fight, ditch the sponges in the fluval. good luck

I am having the same problem in my system with cyano, although it hasn't killed anything, yet. I did turn off my lights for 60 hours straight, and this knocked the cyano down drastically, but not completely. The 60 hours without light had no ill effect on my corals or anemones.
I am planning on a 3 day complete black out shortly to completely get rid of the problem. This means you need to completely cover your tank with blankets to ensure no light gets in. There has been a few threads on www.reefcentral.com on this issue and this has been where I have done my research on how to fix this problem.
You would have had a problem that started the cyano and you could have fixed it, but once it has established, you need to do a 3 day blackout to get rid of it. I would get rid of any media in the canister filter, as this may be one of the reasons this problem started. I have also added extra flow, lowered feedings and changed out my bulbs. None of these affected the cyano at all, and it continues to get worse all the time.
I am planning on doing my 3 day blackout the weekend of September 10 to 13th. I can let you know how it goes if you want to see how mine turns out before you give it a try.

I got rid of my cyano easily by vacumnng with everything off, I cut my lights back 2hs and added 2 koralia2's aimed at the worse areas no probs I left it like that and changed my water schedule to 5% every 3days and wala:):)

I had to do a blackout when I had a bad algae bloom a few years ago same thing usd blankets because even ambiant light will help it I also used weiss algae remover gone in a few days:) not sure if it's safe to use with reefs though :)