I have a cascade canister on my big tank, I have always had a problems with nitrates and doing some research have found that canisters are one of the reasons why... Is there a safe way to remove it without having a major effect on my tank???????
Canisters have the same drawbacks as bio-balls, they're nitrate factories !
Sponges, while great for your water clarity, accumulate nitrates like crazy. If you really want to keep your can as is, the best thing to do is trash the sponge filters and use throw-away filter pad. But, here's the rub, if you want to eliminate nitrates, you need to change it out at least every 3 days. Who wants to deal with that beating?
Joefish has nailed your second option. Slow and steady removal of the sponges. If you have a nice supply of cycled live rock that's been in the tank for a while, this will speed up the removal. Live sand too? Even better. In fact, if you've got a good supply of both of these, I wouldn't even bother with the rubble in the can. I'd just use it to run carbon and circulate water.( Just make sure you test the water thoroughly right before every phase of the removal. If there's any hiccup in the results, wait a few days and test again. )
I have looked around a fair bit about cannister filters, and I would say that there seems to be a lot of dislike about them among the reef community, that may not be entirely fair.
A cannister filter will begin to accumulate nitrates as it can become blocked very easily,- in order for it to work for, not against you it must be appropiately mechanicially filtered to prevent any particles reaching the media.
I know of many successful reef keepers who use cannisters - and do so without nitrates. I am not sure how often they replace the prefilter, I do believe that many just wash it out. A sponge is not recommended as a prefilter, as munkeybutt said, but I'm not really sure what people use otherwise.
Bingo! That's the problem. These things (Eheim's, Rena's, Fluval's, ect.)come with foam sponge filters as standard equipment. These filters need to be properly cleaned or it can snowball into big problem-o's, i.e. increases in nitrate. I've seen people with 2 sets so they can dry one in the sunshine to sterilize while running the other. Don't get me wrong, I'm not opposed to them. They're great for mechanical filtration but they leave alot to be desired when it comes to biological filtration. I just wouldn't use one on a tank bigger than 55g, but that's a personal opinion.
One more bad rap against. (man, it does sound like I hate these things) I have seen Fluvals on 2 seperate occasions heat up in the impellor housing and try to cook an aquarium like a runaway heater.
Ok here is the run down on my tank, Nancy you have seen it, I have a 150, when my LFS closed down i took all the teenagers i could find to help them move out and they gave me the whole show tank full of LR, I was giving it away i had so much. Anyway, I have so much LR now, that i have a 30 gal below the tank i use as part of the sump full of LR and it drains into another 30gal which holds my PS and return. Also in the main tank I have ALOT of LR. Before all this took place i used the canister as a mechanical fitler but now its just a pain in the Butt. Thus why I want to remove the nitrate producing pain it is. OKOK im rambling, to much coffee this morning Thus my question on removal....
If you've got all that live rock its probably already sustaining your tank. Assuming you have a skimmer, I think you shouldn't have to worry about any more mechanical filtration than what it and the prefilter on your overflow are providing.