Hi all, thought I'd give you an update...
I got quite worried over the weekend as Brynn was looking decidedly sorry for himself. Ammonia and Nitrites were a concern and I'd had to carry out constant water changes since Friday. He wasn't eating and because he spent most of his time resting on the bottom even his fins seemed to be suffering.
Thankfully I think I've now got things sorted and he's looking and behaving much better.
Because of the dodgy filtering I took out the air pumped filter and replaced it with the original powered one - with the media swapped back of course. I've left the air stone and sponge one in place though as a nice backup. Levels are now thankfully under control. I'm still carrying out daily w/c's but only 2L and only as a safety net and to get him used to being mucked about.
I added a second 25w heater and between the two am managing to keep the temp at a constant 26 degrees. (I'm sure the lowly 24 degrees I was managing with only one was contributing to his unhappiness.)
Brynn's also eating happily now - spitting the pellets out if they're not soft enough but going back later when they are. Soaking the pellets does work to a degree but the trouble then is they sink and he has to go foraging anyway, so I might as well not bother so he at least knows the food's there.
Because I'm keeping the feeding to one corner (and the filter current tends to gather it there anyway) I've swept the floor of gravel in that area so he can get to any sunken food easily. It means I can easily monitor how much he's eating and clean up the debris too.
Anyway, some updated pics. First up, flaring at his reflection in the mirror I pinched from herself; the overall tank; then the airstone-with-sponge that cost me the grand sum of £2.50...