Unexplained night deaths....lack of O2?
In short, over the last couple years I've had a series of fish deaths that I've struggled with. Very rarely has it been more than one or two at a time; generally when it happens it's overnight. My time to enjoy the tank is later in the evening before the light timer goes off (10pm) and everyone is doing fine. Then in the morning is when I see the corpse(s).
First off, the water tests seem fine. I stock very lightly and have never been one to put a lot of new fish in at once. My current load would be ok for a 10 gal (other than a large algae eater)...the tank is a 75g with two canisters on it, WAY overfiltered but (I don't think) not too much flow (one is an old eheim that really doesn't have a ton of flow).
Other tank notes:
- a couple of my fish have been in there for years and they are ok. I have an amano that is roughly the size of a small lobster, he's been in there for years.
- it's a low-tech tech with (to my discredit) sporadic light dosages of excel. Many of the deaths happened prior to me ever using it, and using it hasn't changed anything.
- I have a number of both root feeding and other plants. That said, it is not a tank that is nothing but a wall 'o plants (yet!)
I have had a few instances of ich and moving forward I am resolved to use a QT tank despite the hassle. I mention this because a couple of the deaths have happened within a few weeks of treatment (higher temps and salt) and I thought perhaps those fish just got weakened. However, there have been similar deaths at times far before or after the ich.
Other things I've considered:
- airborne something bad - I've tried to make sure nothing is sprayed near the tank and no cleaning stuff is near it.
- something bad in the filter(s)...but I've had a HOB on there recently in place of my sunsun that I was repairing... and had two deaths. The eheim has been on various tanks for 12 years and I really doubt it is the cause of any of this.
- type of fish - most of the deaths have been rummynose and lemon tetras. The former known to be fragile, the latter hardy. However, I've lost cories, a couple giant danios and a couple buenos ares tetras (the latter two types holdovers from when I had a few South American cichlids in there).
- water vac disturbance - I've noticed a few times that deaths have happened after the tank was cleaned and water changed...which if anything should be helping the fish. I'm very careful to rinse my hands carefully before touching the water. I thought maybe that "stuff" might be getting stirred up, especially if I've moved a piece of wood or rock that had detritus under it.
Ok, long story short, as I've been grasping at anything I can..
The fact that I've noticed this after water changes made me realize that after water changes the tank tend to be full. I sometimes top it off before a change too. And when it's full, there is way less surface agitation. Could this be o2 deprivation? I haven't used an air pump in...30 years maybe!...but I also have never had near this many plants before. I've read that plants at night will use some O2--even without co2 in the tank, could this be an issue?
I've ordered an air pump/stone to try it out, it can't hurt. I could use it on my shrimp tank too (which has ALSO had a bunch of deaths, though my ONE remaining male apparently came through as a female is berried! Way to go, dude!). That tank has plants as well, and I have kept it mostly topped off (filter is an aquaclear). Conceivably the same issue could be afflicting both tanks if it's an issue at all....
Anyway, this long-*** post brought to you by an extremely slow work day, thanks for any words of wisdom!