Erin, I have two more suggestions. They may not sound ideal but they are options at least. One is to go green. Before you say "AAAgh I'll never see my fish again" give it a thought. Believe me, it is a sure fire way to get
zero nitrates if you can have a window that gets bright light and you can place both tanks under it. GW is incredibly
easy to manage. It really helps with floaty fish and it gives you amazing
peace of mind. Priceless. Water only needs changing once a week.
The other one is more aggressive filter cleaning. Anytime you have a lot of fish producing a lot of waste theres going to be more nitrites and ammonia for the bbs to break down and de-nitrify resulting in huge nitrate levels. Your bb colony must be
huge, managing to make all the nitrates..perhaps cutting back on feeding (more frequent fasting days?)
PLUS new filter sponge or at least a good rolling boil of
some filter media
in one filter only half the material at a time over a few days if you are worried..might help. Along with the fasting it would be fine to cut back your bbs some. With my tanks that have been set up for ages & ages, nitrates creep more and I need to do much more aggressive filter media cleaning. It goes against traditional thinking to consider cutting back bbs but if you can get the balance right with less feeding at the same time it is safe in a well established tank like this. Of course you have to be very careful -to slowly increase feeding etc -but I know you would be. Its not a permanent solution but it can give the fish a break from high nitrates for a wee while.
And the other one is, like Andrea suggests, a commercial helper like Nitrazorb in the filter. I think they are like carbon and need replacing and will release back but one every 10 days (you'd have to monitor how quickly they are used up).
We're with you, hang in there