okay - here's the scoop. hubby is finally convinced to do a fishless cycle. ammonia is NOT low - he did a strip test. The liquid test and the LFS liquid test show that it is above 1.0. Nitrites are starting to rise so he is hoping for 2-3 weeks, but not dead set on that date, for it to finish iinitial cycle. Adding a pinch of food a day, stopping water changes (is this good?), turned the heater back up to 78 (had lowered it after the last fish died to make it inhospitable for the columnaris), and just wait - with periodic water tests.
On the lighter side, while we were at the LFS, we found a 55 gal tank and stand, just the tank and stand (well, the light too, but it's not really the right size), no top, filter, heater, etc. for $60. bought it to slowly buy the needed components, with great care and careful selection, and eventually (hopefully in a month or two, have it filled and cycling itself. Anyone have any thoughts on types of filters for this? He wants to put in at least 80-100 gal capacity filter. It is long, so would it be a good thing to put a 50 gal (170 gph) bio-wheel filter on each end?
So, the danios will have to wait - in fact we have decided to spend the cycling time on the 20 gal (and the 55 gal) researching fish types, compatability, etc. and carefully select our fish, adding them much more slowly and no more fishy cycles.