Instead of chemicals or creatures, why not not pick up a Hang on the back filter. It will help pull the junk out of the water safely and without adding any thing else to your tank. You don't want to add another "creature" because your tank is already have problem handleing what is there all ready, it doesn't need, and may not be able to handle anything else. It could send your tank into a crash. As far and "chemicals", All most of those will do is detoxify the toxic nitrates, not remove them. Then you will have a hard time telling what is really going on in your tank. Also it not a good ideal to add anytype of chemicals to your tank that you can not test for. There is not way of knowing how much of anything is in your tank. I haven't dealt with UG filters, so I might be corrected, But what I would suggest to you is:
1). Get some new fresh RO saltwater ready. and an extra small tank, bucket, tote something that will hold the fish for a few days.
2). Pick up a Hang on the back filter, You can pick one up at your local pet suply store. wal-mart, just try to stay away from the Whisper's if you can, they are basically junk, but if that is all you can grab for now, it will do for a short time.
3). Carefully get out as much of the water from the tank you can without disturbing the sand. Put this water in your buckets. Put the fish in the largest bucket or tote that you have. Put about 3/4 tank water, and the rest the fresh RO saltwater. So it will be like doing a water change. The fish should be fine for a few days in the bucket or tote as long as you have circulation in that bucket.
4). On the main tank, try to slide the undergravel filter out from under the sand. Try not to disturb the sand, it will be difficult, but if it happens, you shouldn't have too much of a problem.
5). After you have the undergravel filter out. Level out your sand, and take out the most water you can without disturbing the sand anymore. Put your rock back in the tank and the water that you had taken out before you messed with the filter. This water will have alot of the bio you will need. You would have all the water because some of it is with your fish.
Just use more fresh RO saltwater to top the tank off.
6). Put your skimmer and new hang on the back filter on the tank. Let them run for a few days. You may have another small cycle but it shouldn't be too much, or last too long. After a few day the tank should be settled down. You can start adding your fish back into the tank. Just add a few every couple of days. It really shouldn't be any different that if you moved your tank from one side of the room to the other.
The only thing to remember about Hang on the back filters is, they work quite well if maintained properly. The filter pads will catch alot of the junk in the water and the carbon in the pads help the break down the toxins. But the filter pads must be changed regularly. The junk builds up in the filter and you will have a nitrate factory going on again. Depending on your livestock load in your tank, you need to do your water changes and change the filter pads every one to two weeks.
