As an update on the above posts, Huge Cory lost the remainder of his tail about a week later - I witnessed another pygmy cory bite it and another section out of the other cory's tail. Since then, he has remained hyperactive but was lying completely on his side on a plant leaf one day last week and still hadn't moved several hours later or when I lifted the tank lid, so I went to remove him, only for him to torpedo round the tank and resume his hyperactive swimming up and down the glass; I can only assume that a fortnight of hyperactive swimming had worn him out and he was taking a much-needed rest. However, this morning, he was struggling to go up to the tank surface to take a gulp of air, and was rolling around quite a bit when trying to swim. On return from work, he was doing the same, and also looked very washed-out like Dead Cory looked in the couple of days before he died 2.5 wks ago, so I immediately moved him into the small container and noticed that his tail stump is slightly red, so have treated him with a generic medication (Aquarium Munster Medi Mor, which I bought from my LFS as it doesn't sell the eSHa products which folk here generally recommend) which treats fin rot among other conditions.
Quarter Tail Cory seems to be fine although has had to adapt his swimming slightly, mainly to set off from a sitting down position.
Two cories, both long and thin, and one of which is the biting-tail culprit, seem to be fine - I'll refrain from naming them Healthy Cory 1 and Healthy Cory 2, to prevent tempting fate.
Bent Spine Cory with the white fleck which comes and goes (usually but not always on a Thurs, but which disappears on a Fri - before the Sat'dy water change, so not connected with building nitrates) has been doing fine but was lying on a plant leaf, completely on his back/upside down, yesterday afternoon; after waiting a while, when I went to remove him, he torpedoed off round the tank and seems to be fine although the white fleck is evident. I'm wondering if he may have found that lying on his back actually provided some relief for his bent and presumably weighty spine.
My questions therefore are:
* do I set up the QT this evening and move Huge Cory into that to treat him? [I don't want to leave him in the small container with only 4L water and no filter or heater for too long. I can move one of the two filters across into the QT and have a spare heater. I think he may struggle to reach the water surface to get his gasp of air until he's feeling a bit better.]
* do I move Bent Spine Cory and possibly even Quarter Tail Cory in with him to treat them as a precaution (or maybe even all 5 cories), and give him some company?
AND/OR
* do I treat the entire mainstream tank (which houses the x-ray tetras and harlequin rasboras)?
NB. Water readings have continued to remain exactly the same as per original post, even with the addition of the harlequins recently.
Thanks in advance for your help.