Goldfish pop eye?

[CR45]

Hi, I have an approximately 15 year old comet goldfish that’s been having some issues with lethargy and pop eye and am looking for some advise.

He’s had what looks like pop eye in right eye for a few months now which does not seem to be improving. Over this time he has become more lethargic and rests on the bottom of the tank a lot typically hiding behind one of the plants. I’ve also noticed he periodically gets red streaks in his tail and sometimes red “sores” - I did think this was either ammonia burns or haemorrhagic septicaemia but ammonia readings are okay and I would have though septicaemia would be fatal by now??

Tank parameters (api test kit):
pH: 8.2
Ammonia: 0
Nitrite: 0
Nitrate: 40ppm (seems to fluctuate between 40-80, tapwater around this range too)

Currently in a 72 litre tank that has been set up about 4/5 years. I realise this is small for a comet goldfish but previously he was in a much smaller tank (sadly down to my previous lack of knowledge) and so is stunted unfortunately.

Last water change 2 days ago, changed 50%. Used fluval water conditioner. Typically try to do 25-50%change weekly but more at the moment due to fish Illness depending on meds timings

Ciano CF80 filter (Also had elite stingray 15 filter until started medicating tank and needed to separate snails!). Also tetra air pump

Single comet goldfish and 2 nerite snails

No changes to the tank recently apart from adding live plants about 6-12 months ago.

Fed tetra goldfish flakes once daily. Did try switching to pellet type food 12 months ago but fish did not get on with it.

Treatments already started:
- Melafix (2 week course but mistakenly left in carbon filter!)
- interpet anti-internal bacteria (on day 8 of treatment)
- nt labs tonic salt (started 2 days ago)
- tetra nitrate minus (started yesterday)

Based in uk so no access to overcounter antibiotics as far as I am aware



Sorry for the long post. Any help would be appreciated