One has enzymes the other has live bacteria.

So, my questions relate to the Pure aquarium balls, Pure also do a pond version which is a black ball, while the aquarium one is a clear ball.

What is the difference between the two?
I assume the pond ones can be used in an aquarium?

Thanks so much for the great reply - I'm happy to hear that he might not be suffering, so will continue to care for him.

It may be that the oranda is perfectly happy living a more sedentary/sedate lifestyle, conserving its energy for feeding time. If moving before feeding as you mention, this suggests that it still has motivation/interest, and it doesn't sound otherwise distressed. I definitely would not euthanise it but simply let it see out this chapter of its life in what must have been a lovely home for the past decade.

We have had our Oranda for about 10 years now and over those years there have been one or two issues (fin rot, a large irregular growth from the wen) that I now think are limiting the quality of life.

Now, it mostly lays on the bottom of the tank, except before/during/after feeding - I think this is due to a combination of the historical fin rot, the size of the irregular growth and perhaps loneliness, although in hindsight, this inaction seems to have been triggered about 18 months ago when the temperature in the tank dropped very low and I decide to change the tank completely.

I'm just wondering if I should euthanise it or keep on while it's still interested in feeding?

If it were me I'd do two 50% water changes back to back and then treat.

Often new fish do this as they are understanding their boundaries, exploring etc.

You need a plec. It’s covered in some sort of green algae.

That’s interesting reading about salt and medication. Many meds I’ve seen suggest adding salt. Why would you not recommend this?

Poor Apistogramma