Hi, thanks so much all. Will do water change asap (but have a few more questions before I do anything wrong) and make sure to use the gravel vac on the substrate - I wasn't worried about the muck it as I was under the impression that although it looks unsightly (wasn't a huge amount of muck though) it was good for the ecosystem, didn't realise that it could cause this. I normally always clean out the sponge but haven't done the pump itself for a few weeks so will do this too.
I forgot to mention I've been having problems with nitrates in my tap water, I've recently purchased a pozzani tap water filter and done everything right as far as I know - ran the hose through for a few minutes first to wash out softening agents, ran the filter through itself for five minutes before filtering water for my tank as instructed, tested filtered water before putting it in my tank. Could the nitrates have caused my fish's illness?
And my main question - the pozzani filter lowers my pH from ~8 to 7.2 - but is necessary due to the high nitrate content of my tap water, plus I believe the lower pH will be better for my fish. My last water change (sunday) was a smaller one as it was my first time using the pozzani filter and I didn't want to drastically alter the pH of the tank all at once. So for my asap water change, what should I do? Should I do a large water change using the pozzani-filtered water and risk changing the chemistry of the tank water significantly and stressing the fish? Or should I go back to my old method whilst this fish is unwell which would mean adding in water high in nitrate content, but not different to what they're used to and not a shock? Bit of a catch 22 and I'm totally new to illnesses like this therefore I'm feeling very lost!
I did however manage to get some relatively clear photos today:
View attachment 88938 View attachment 88939 View attachment 88940
As you can see the eye is very red and swollen-looking, plus some patches of scales appear to look pale and discoloured. I'm also wondering if the pale thing pictured a bit to the bottom left of the eye could be something, or possibly just a clump of sand (was hard to see up close without the camera's zoom). I imagine it more likely to be something relating to this.
Also I feed my fish flake food and two algae pellets a day (bristlenose pleco, 4x bronze cories, 3x zebra danios, and a red wagtail platy) - been looking into frozen foods for them but will hold off if advised whilst the fish is unwell (I assume not good because could add more bacteria into the tank)?
I hope the pictures can bring some clarity to this - could my other fish catch this? I'm worried and just want to make my poor cory cat better. Would really appreciate some more help please and answers to my questions - in particular the one regarding which water to use for the water change/s whilst my fish is unwell. Please and thank you!