Hello; I just read thru this thread. My understanding is that you do not have a second tank and want to keep beneficial bacteria going while you change over to a community tank with entirely different fish. Do you have a bucket, perhaps used for water changes? If so, put some gravel, ornaments, heater and if possible run the filter from the current setup in the bucket with a few hardy fish (I usually fill the bucket with water from an established tank as well). (My filters do not fit on a bucket rim.) If you have time run, a sponge filter in the current setup long enough to establish the bacteria. I used to have the habit of running a sponge filter buried in the gravel of a large tank and would throw it in a tempory tank while redoing a tank setup.