Also when you kill off cyano
or other nuisance algae en masse like that they will rob the water of oxygen as a part of the respiration process when they die. Or rather overload it with co2
. Anyway they can create deadspots, so that is another reason to siphon out as much as you can prior to treatment, and additionally making sure there is plenty of surface agitation to keep ph levels stable.
is pretty common when you return from a trip I feel like. It has happened to me a dozen times, the good news is it is usually related to an over feeding by the sitter, or something along those lines. (Tank like was turned on by the sitter and they forgot to put it back on timer, etc...) A water change and a good manual cleaning usually gets the tank back in shape fast, it is the problems that have slowly crept up over time that are the hardest to remedy, I think yours will rebound quickly.