Here is an image that I found, it kind of reminds me of a skimmer that works with sand instead of bubbles, and that removes Ammonia and Nitrite instead of organic waste.
Take a look:
Supposed to be far superior than any wet-dry filter setup in the removal of ammonia and nitrite.
I have never had this type of filter before thats why I ask for your opinions.
Fluidized sand filters are very good at processing ammonia and nitrite, but since they are closed filters they remove oxygen out of the water during the nitrifying process whereas wet/dry filters use atmospheric air instead.
Of all the fluidized sand filters, the Rainbow Lifegard units are my least favorite. They would routinely back siphon when turned off for maintenance on the tank. The check valve they come with didn't work and the sand would back up into the pipe work and clog the whole thing. I
became so frustrated with them that I ended up taking them off all the systems I maintain and throwing them in the garbage.
Sorry to be the bearer of bad news. Perhaps you will have better luck with them.
My skimmer oxygenates my tank plenty so I don't need to worry about that, what I am wondering about is what you said about the Check Valve. How long ago did you have one of these filters, was it recently?
I have also heard that these type of filters may sometimes have a hard time starting up if the sand was allowed to settle in the chamber (turning the pump off will do that) and therefore you have to give them a shake to help the pump get the fluidized bed started. Without the shake it is almost impossible for any powerhead to generate enough pressure to get the sand flowing.
I used this model several years ago when they first came out. I could have shaken them for a week. Once the sand back siphons into that little channel were the pump input water comes down, you can shake and shake, but in my experience I had to take it apart and get all the sand back out before they would restart.