well, oddity of oddities...
I cleaned out the bugs tank - it had 2 doses of snail rid, and multiple fresh water changes with the filter sponge and the driftwood over a few days. I figured everything would be dead, probably including the filter bacteria as it sat in a bucket of tap water for a few days.
So after being put back together as a cory tank the water is now clear with no peat, sand on the bottom so the pH is quite different to before.
Earlier in the week I saw what must be a copepod zapping around, and the little white worms/planaria are back on the glass. All these critters had disappeared when the killiefish were in there, I presumed they had been eaten.
So today out of nowhere I found an ostracod! It could only have 'hatched' from the filter (if they do such a thing) after not being killed by snail rid and untreated tap water, or have been embedded in the wood somewhere and now emerged.
It's unbelievable. And the shrimps are still going strong too.