I have a 3-year-old 10 gallon tank that I have recently, within the past 6 months, dedicated solely to Otocinclus catfish. I absolutely adore them, and for me they've been the trickiest fish to try and keep happy. I've never been able to keep a whole group alive, and I've usually blamed this on most specimens being wild caught, and the transition to pet-life being too much on such a sensitive fish. I'd recently gotten a group of 10 and had zero deaths in 2 weeks! I was ecstatic.
I had a breakout of blue green cyanobacteria in my tank and figured it was due to low water movement in the tank and not enough nitrates [my ppm is usually between 0-5ppm because it's a planted tank]. This tank has always had blue-green algae, but it just recently got a bit worse than normal, likely because of the extra load of fish. I got Ultralife's Blue Green Stain Remover and an airstone, put the recommended dose in for a 10-gallon and for about 5 days watched the slime disappear! Awesome! I had 1 otocinclus death, but I could tell it was because this individual was not eating. After he started swimming like a corkscrew, I decided to put him to sleep because I've never had an oto come back from the "corkscrew of death".
It's day 6 today and I wake up to a mass death event in my tank. 5 otocinclus are dead. I immediately do a water test [7.2ph, 0 ammonia, 0 nitrite, 5ppm nitrate] and did a 20% water change and put some prime and stability in just in case before the tests were ready to read. What could have possibly happened overnight?
The remaining otocinclus seemed extremely lethargic and not active at all. Since the water change [~20 minutes ago] they are mildly more active, but hardly. Before, they had all been swimming in little groups, happily eating algae and biofilm and were fairly active.
I don't think it was a lack of oxygen because my couple of ramshorn snails, assassin snail, and one nerite weren't all crowding the water line like the water was low on oxygen. Also, the tank has an airstone.
I definitely don't think it was starvation because all 5 died at around the same time and 2 look nicely plump with the other 3 looking a normal weight. None of them look bloated.
I know quarantine period is usually a month - could there have really been some disease waiting to strike? It seems unlikely! But I'm not seeing any water parameters off.
I don't have any tests for KH and GH, could hardness really be a huge issue overnight?
I really don't think it was ultralife's fault because if that had been the problem, I think I would have seen behavioral changes 6 days ago.
Any thoughts?
*Edit*
It's about 5 hours later and activity level of the remaining 5 is totally back to normal. I did end up testing the hardness of my water [after the 20% water change] and it was a bit hard at a GH of 300. This makes sense as I used conditioned tap water for my tanks - but I don't believe there would have been a sudden change in hardness overnight. I'm still so confused. I might get some RO water from a local fish store and see if I can slowly bring the hardness down since I know in the wild Otos prefer soft, lower PH water and right now I have hard, mildly basic water.
*Editing to add photos of the tank, otos to show body condition*
