Re: Massive water change
#2
It sounds as though the cycle has 'crashed'. 90%+ of the beneficial bacteria is housed in the filter media itself, and a comparatively small proportion on the decor and in the substrate (especially if the gravel layer is thick). The full water change per se is unlikely to have caused a problem but the filter failure and subsequently the stirring-up of the gravel (and potentially pockets of gases in it) are almost certainly the problem. The trace of ammonia and the 0 nitrate (ordinarily nitrates ought to be a bit above that) almost certainly confirms this. Follow the advice in this
https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm as it's essentially the same as what needs to be done for the fish-in cycle that you're now faced with.
Keep monitoring ammonia, nitrite and nitrate daily for the next 6-8 weeks until you get through that cycle. Don't, under any circumstances, be tempted to get any more fish during this time.
If the last 3 fish survive the fish-in cycle, you'll need to seriously consider a bigger tank that meets the requirements of the fish you have left (check
https://www.fishkeeping.co.uk/modules/ ... sheet.php?caresheetID=100 and
https://www.fishkeeping.co.uk/modules/ ... resheet.php?caresheetID=1 and
https://www.seriouslyfish.com/species/trigonostigma-heteromorpha/) and adding to the shoals as required, etc.
Hope that helps.