You have confirmed my suspicious and worries about Ping and the tank.
Based on your history alongside the odor, it sounds like you are facing an issue with nitrogenous waste (nitrites, nitrates, ammonia) accumulation in the tank. The black discoloration you are seeing is a sign of high ammonia concentrations burning his skin/scales. The strange swimming (which his ability to swim normally when necessary tells us that the swim bladder is not an issue), depression and bottom sitting are all signs we see with nitrate toxicity (since nitrates bind the hemoglobin in the blood leading to poor oxygenation and therefore weakness, disorientation, and lethargy in fish).
Now we can see tank water balance crash for a range of reasons. If you have not been making any changes in the tank (ie new fish, new filters, etc) prior to this, then the most likely cause will be that there has been a crash in your biofilter (the good bacteria that break down nitrogenous wastes). And while the water change would have reduced nitrogenous toxin levels, we do have to be aware that full water changes can further reduce residual biofilter and can also lead to crashes in the affected fish (since nitrate toxicity needs to be weaned down slowly to avoid shock). Furthermore, this situation is likely being exacerbated by the excess food that will be breaking down and further contaminating the water in the wake of a low residual population of good bacteria (though bad bacteria could choose this time to bloom -- this can lead to a rotten odor, water clouding, and white strands of bacteria in the tank if allowed to develop).
In regards XXXXX XXXXX a situation of this nature, it would be ideal to check water parameter (ammonia, nitrites, nitrates, pH) at this stage. This would give you a baseline to work from and compare parameters as you try to restore the tank's balance. If this is not an option (if you don't have a kit and the local aquarium shop isn't open), then we'd want to start managing the tank until we can check these. Now the primary way to remove nitrogenous wastes from the water is via water change. It is worth noting that we never want to do a full water change, always partials (even if it means doing a few partials over the course of the day). The reason is because as I noted we can see this kind of issue rise when people do full changes (its very common) and therefore destroy their tank biofilter. Nitrate and ammonia binders can be used, but freshwater is best, XXXXX XXXXX has the added bonus of more fresh oxygen then treated water. Good quality water and great oxygen water saturation is important in these cases because nitrates cause problems with oxygen exchange at the cellular level.
Therefore, in this situation, you would want to consider starting on partial water changing this tank. Ideally, we want to spread a water change of 50% over the next 24 hour period. Small changes frequently in these cases are better then large, since nitrate levels have to be decreased gradually (to prevent further shock for Ping). Make sure that the water is treated with dechlorinator and that its room temperature when you add it (to avoid temperature shock). Furthermore, for the future, if this tank is struggling and the wastes are in delicate balance, you can consider a routine 20-30% water change 1-2 times a week to keep the nitrate levels in check.
If you are using a heater, consider reducing the water temperature since this will decrease his oxygen requirement. And if Ping is showing any huffing and puffing, then consider increasing your aeration (ie add another air stone) to make it easier for him to oxygenate. As well, you can add aquarium salt is here to support him at a low dose (ie 1tbsp per 5 gallons) to support him during this stressful time. And if he is leaving food, do make sure to offer small meals a few times a day (to make sure nothing is left) and consider hoovering out any left over rotting food to avoid it thwarting your work to restore the tank balance.
Finally, once your pet store is open, do consider looking into getting a biofilter bolus (ie Tetra Safestart - example). This will restore your 'good bacteria' population quickly and these bacteria will help break down those nitrogenous wastes and aid you in settling this tank crisis.
Overall, the signs Ping is showing and the tank's history does support a tank crash and a dangerous elevation of nitrogenous waste (especially ammonia/nitrate) concentration in the water. Therefore, you do want to take the above approach to address the tank issues and as soon as possible you will want to check your tank parameters (though hopefully by morning they will be approaching normal) and consider bolusing the bacterial biofilter to make sure it is at optimum to help break down these nitrogenous wastes as it should.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
If you have any other questions, please ask me – I’ll be happy to respond. Please remember to rate my service once you have all the information you need. Thank you and hope to see you again soon! : )