Thanks for the replies everyone.
I do realise that low tech tanks seem to benefit from an "active" substrate such as added soil etc. However many people grow great plants in straight sand. And there's a good buildup of mulm in there. I was planning on getting some red clay from the river here, mixing with some soil and sliding under the sand to see if that would help (freezing or drying it first). I'm kicking myself for not coming across Walstad's book before setting up the tank. But even plants that were left floating seemed to have problems growing. I've had thoughts of switching the substrate to pool filter sand as it's pretty much guaranteed to be almost pure silica. But the river sand wasn't a problem in my old tank, or my friends.
As for the lights, they are NO fluoros. I thought the old bulbs may not be good enough and replaced them a few weeks ago with the better ones. It's DIY lighting and the fixtures have pretty good aluminized reflectors in them. It looks a fair bit brighter than my friend's successful tank even at substrate level.
Ferts: I was dosing a macro mix up until a couple of months ago, when I thought that could be the problem. If I could get the plants to at least grow with that stuff my plan was to get some pure macros. But I'm not wasting money on them until some success comes my way. And I expected the fish load in there now would take care of most with the amount of light. At one time it was suggested low potassium might be causing problems, but dosing K2SO4 didn't help either. And I'm pretty sure excess nutrients can
cause problems with uptake of others in some circumstances.
Also, I haven't got a phosphate test kit. I'm a little scared to dose that since I haven't a clue if it is deficient or maybe excess. IIRC, excess phosphorus may lead indirectly to Fe or Zn deficiency. Maybe that is the problem?
So my options seem to be:
1. Pull tank down, start from scratch with better substrate
2. Begin EI dosing and hope for the best.
3. Let the tank have a rest, water changes only for a while to see if it gets better. Also give the substrate a good clean out.
4. Swap the tank for my friend's
4. Forget about plants
Any other thoughts/ideas?