Help needed. We have a 30l tank that had 3 Otto's, 3 rasboas and three tetras. Had been ticking along quite nicely. I've been poorly and so haven't been in charge of taking care of the tank for a month or so. When I came to it last week the water was really really murky. I went to do a serial water change as an urgent mesure And noticed the filter was making an odd sound. Long story short, it wasn't working properly. I managed to get a new filter but there was loads of waste in the water/gravel and so I decided to do a full water change. I rinsed the gravel lightly, put some of the filter media from the old to the new and set it all back up.
I am now down to one tetra and 2 Otto's. The rasboas were gasping at the surface the day after the change.
Do I look at it as a new tank and cycle again? Or is there a specific way to deal with the tank from this point on.
Oh I have been testing the water daily since and trace ammonia (did a small 10% change) no nitrate/nitrite showing
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.