Thank you for the photo of your wee fish.
I appreciate that fish are not the ideal model and a close-up of the eyes would likely not be possible. Still from this angle, it doesn't look like we have any external infectious growths on the eye. Therefore, I would suspect that the dullness of the eyes may be a part of his overall ill thrift at the moment or an early sign of corneal edema secondary to bacterial opportunistic agents (which we will treat simultaneously with the fin rot).
Now I didn't see a response from yourself regarding those last 2 wee questions. Still from the history and your treatments so far, there are a few concerns that we can appreciate. First, I am curious to when
the water parameters were checked (before or after the water change). If they were before, then the dramatic change will likely have made a dent in the borderline nitrate levels. If the reading was after, then this would suggest that there was significantly higher levels before and that the water is only now borderline. If the former is the case, then it'd be worth a wee check again to make sure all is in line now. If the latter is the situation, then water water changes need to be continued (see my example below on how to do so). As well, you didn't note an ammonia level, but do know this is an important one since it is usually the first nitrogenous waste to increase in the water.
That all said, I must say that the partial water change was an aggressive one, especially if you have also thoroughly cleaned all the tank components and changed the gravel at the same time. The reason why more is not always better in fish tanks is because this will have not just addressed any water parameter issues but will also potentially knocked back the tank bacterial biofilter population (the good bacteria that break down toxic nitrogenous wastes). And that means if they were struggling to break down nitrogenous waste before, then they will struggle even more so post-water change and we can see dangerous nitrogenous levels become an issue again even quicker.
As well, big water changes can actually harm fish too. For example, if we are facing a nitrate toxicity and need to carry water changes, then fish need to be weaned off the high nitrates to avoid fatal shock. So, when we address these we tend to do small partial changes through the day. For example, we’d start with a 20% change and then aim to change 50% divided into smaller fractions over a 24-hour period. This then allows us to wean the fish off the high nitrate levels (avoiding shock --which could be part of the issue for the red cap currently) while getting parameters back into a normal range. Of course, it goes without saying that the new water needs to be dechlorinated and be at room temperature (same as the tank) before adding since cold shock can cause also bottom sitting and even mortalities for some fish. And if you are making major changes as you have with water/gravel/filter sponges or carbon, it would be ideal to use a biofilter bolus (example) to ensure you don't decimate your nitrogen-fixing bacterial population.
Now even though the current nitrate reading is in between a normal (10) and a toxic reading (25) (since we want this parameter as low as possible but absolutely no more then 20), I would still be concerned that it could be playing a role here with the signs you have described for this red cap. The reason is because we often see nitrate intoxication start with the fish intermittently bottom sitting. This happens because the nitrate binds the red blood cells preventing oxygen uptake. If you imagine this makes the fish weak, tired, dizzy, and ‘out of breath’ quicker. And this leads to a fish who can swim normally but gets winded quickly and ends up bottom siting while they struggle to equalize their blood oxygen levels. As the intoxication progresses we can see increased respiratory rate and distress/gasping (which could be related to the bubbles). In advanced stages, they will sometimes assume a bent/curled positioning (think of a diver with the bends), show a crooked spine; uncontrolled swimming, twitching, spasms/neurological signs, and loss of appetite. Furthermore, if we have fish who are experiencing low grade chronic intoxication, we can see milder signs of the above and these fish would be prone to secondary health issues cause by opportunistic agents (like fin rot).
That all said, no matter the trigger, we need to also turn our attentions to the external clinical signs you are seeing with these fish. As I am sure you can appreciate these are as important as potential tank issues but can often arise secondarily to tank issues that may be triggering stress for the fish (as stress hormones then dampen the immune system’s ability to prevent these kinds of infections.)
Just as an aside, I have noticed that you were feeding the fish peas. While this would be indicated with swim bladder disease induced by constipation, this is unlikely to be of benefit here. From your history, it sounds like their buoyancy is normal (so they can swim normally if prompted) but that their issue is lethargy/weakness that keeps them bottom sitting instead of swimming.
Now the white fuzzy material on the fins is a serious concern and is either fungal (ie Saprolegnia fungus) in origin or bacterial (ie. columnaris). It is worth noting that in some cases it is possible that both can be involved. Furthermore, we have to be aware that both of these agents are opportunistic (taking advantage of situations like this) and are quite contagious. This means we do need to make sure everything in the tank is right while initiating aggressive care to nip this in the bud.
Now if only the red cap is showing signs, then we do have to consider at this stage that it may be best to move this one to a hospitalisation tank. This will allow you to take him away the potential tank stressor, monitor him closer, only treat him with medication (since we don’t want to treat the other if not necessary) and avoid any strain treatment may put on the main tank. If you choose not to do this, then remember that you will need to take the necessary precautions with your biofilter, any live plants, and be read to continually monitor the water parameter levels throughout treatment.
In regards XXXXX XXXXX for the fin rot, we do have a few options that we can use (often choice tends to centre around local availability and time constraints if they need to be ordered in). If you do need to treat in the main tank, then ideally you want to use KanaPlex (Kanamycin). This antibiotic will target the issue at hand and tends to be fairly gentle on tank biological filters. That said, if you cannot source this or get your pet store/vet to order it in for you, then you could use erythromycin or tetracycline (though these are known to be harsher on biofilters and can affect ammonia levels). A third option that would be acceptable for treating the red cap would be nitrofurazone.
If do choose to use Kanamycin in the tank, you may also want to consider supplementing with Melafix. This is a natural antibacterial made from the melaleuca tree and can also be helpful to cover against any fungal agents present.
Overall, the external signs are likely secondary issues for this fish. They do need treated as I have noted above to avoid them becoming a bigger issue (ie fin rot bacterial spreading into the blood stream). Still at the same time we need to make sure that the tank is operating at ideal water parameter levels. If there is any doubt, then do consider continuing small partial water changes to ensure good quality water is present for them (I would say to aim for the nitrate reading under this dubious 10-25). Furthermore, while treating this fish, do consider increasing oxygen supplementation for this fish. If he does have low blood oxygenation, increasing water oxygen levels will aid him in breathing comfortably (hopefully leading to less bubbling at the top of the tank) and get him more active for you (which hopefully will get him swimming more and eating).
I hope this information is helpful.
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