At long last, it look as though I have ferreted out the cause of the persistent water stagnation which I have now long been fighting..... Over the time that ensued I observed that the increasing stagnation has always been accompanied by very heavy clouding of the water and rapid heavy accumulation of bacteria scum in the filters. It looks as though the bacteria explosion is depleting the O2 in the water, as the fish began showing signs of O2 deprivation - thus making the water stagnant and odoriferous. Well, at any rate, I will soon have to break down the setup, QT the fish, and temporarily store it in another room while the flooring is being replaced, so I will then remove all the bacteria and start over afresh without using any rapid cycle starter. Enigma solved at last, I hope !