Alan Silver wrote in message

The light bulbs are probably too old, having been there since the tank
was used before, so that could be one issue, although I'm not sure why
it would suddenly affect things.

I don't think so, Tritons are pretty good and have a very long life
span vs most bulbs(90% till they fail). You can swap one out for a
cool white cheapy bulb and see. A mix of these two bulb types looks
nice also.

I had a DIY CO2 kit in there, but I took it out as it didn't seem to be
making much difference. In the good old days it helped, but when the
tank started its problems, I took it out.

My bet is that your tank had enough macro nutrients and then after the
plant mass gfot so big, the plants removed all their source of
nutrients, basically the plants ate themselves out of house and home
after they got too big.
Swords are especially good at this since they are not pruned like stem
plants and kept at a relative constant biomass.
All the CO2 in thwe world will not make up for a lack of NO3,K+ or PO4
etc.

To fix: KNO3, traces, KH2PO4/Fleet enema and water changes(50% weekly)
will do a world of good. Add the DIY CO2 back.

A typical routine would be 50% weekly water change, prune trim first,
then do the water change. Refill tank, add 1/4 teaspoon of KNO3, one
to two drops of fleet enema(PO4), 7mls of trace afterwards and then
once more more 3-4 days later. Keep the CO2 up high.

Main thing to test: pH to make sure CO2 is good.
2x a week dosing of 3 things. Once a week water change.

That's not too bad.
If so, we can turn this into a non CO2 tank also.

Let me know if the above routine is too tough. It will not cost much,
KNO3/fleet enema/CO2 are very cheap and available in the UK. You have
enough light, Traces already etc.


Regards,
Tom Barr