Hello; I did a similar thing before. I had an established tank with a lot of rooted plants. I switched to one of the new, at that time, HOB (hang on back) power filters. I left the under gravel (UGF) in place and turned off the air.
I did notice at one point one of my kuhli loaches swimming down the tube where the air bubbles used to exhaust. The kuhli eventually was accounted for in the tank and I replaced the broken grid cap on the open tube to keep it and any other fish out.
I was not all that pleased with the operation of the HOB filter, as it would lose function for a variety of reasons. (The newer models are much more reliable and simpler to operate.) Some time later I did turn on the air to the under gravel as a reliable supplement.
I have seen posts talking about problems with being able to clean under gravel filters. I do not quite understand what is meant by this, as I have never tried to clean one while it is operating in a tank and under the gravel. As far as I know, there is no need to clean one. There were some other negative comments about UGF's, but the comments were vague and not specific about what the problems actually are.
I have begun to suspect that UGF's are considered to be somewhat old fashioned or out of vogue and not that there is any serious problem with their function. I used them as primary filters for a lot of years and then as part of a system along with some sort of HOB filter. The HOB type does actually catch tank detritus in a removable filter medium. At any rate over the many years that UGF were in use, I do not recall any serious problems.
