I agree with Krish, cannister filters can be a pain.
First, I would get a good skimmer like the AquaC Remora w/1200 MJ. Then, you should assess if you've got enough flow in your tank. You should aim for at least 600 gph flow, more would be better. Then, if you don't do it already, I would blow off gently the rocks with a small powerhead or turkey buster prior to water changes. Then, you should assess if you don't overfeed. Then, if your nitrates are still high, I would give a denitrator a try, which is easy to build and at your nitrate level would kick in a few weeks. Eventually, if everything else fails, you may have to 'cook' your LR if it's old and probably saturated or deal with the SB if necessary. Also, in the long run, you could add a hang-on refugium if possible.
karl