Hi, first post here so hopefully this hasn't been answered somewhere else - I am at a bit of a loss.
6 months ago I set up a 40g Tall aquarium at my office. Thick layer of fine gravel, driftwood, fully planted - beautiful tank actually. I was breaking down my 180 gallon at home and kept my two favorite fish, a 7 inch boesemani rainbow male and 6 inch female. The male was spectacular. They were quite happy in the tank here, after 4 years they were finally starting to display mating behaviour. I had to go out of town for a month on work and while I had asked a coworker to keep up with the water changed while I was gone (the tank had just finished cycling) they didn't, and I came home to the most insidious beard algae outbreak I have ever seen. My LFS recommended overdosing the tank on liquid carbon, flourish excel to be exact. I had done that before years ago in a smaller tank and it basically cured the algae issue with no other ill effects so I went ahead at his recommendation of 3X the listed dose on bottle.
Imagine my heartbreak the next day when I came in to find both my incredible, fully mature Boesemani's belly up. They were the only things that died.
Anyway, since they were the focal of the tank, I decided to try out Discus to replace them. As of now, I have 4 medium sized discus in their (2 months now) and they are thriving. Everyone in the office loves them. They're with assorted tetra's, cardinals, a whiptail cat, a male beta and zebra danio's and two blue dwarf rainbows and a dwarf gourami.
I had to crank the temp on the tank for the discus, but I'm still in love with boesemani's. About a month ago, I was at a different LFS and they assured me that most rainbows can succeed at the low end of the discus temperature range (I have the tank set to 83 degrees and the discus seem fine) so I bought 4 baby boesemani's, two females and two impressive males.
everything was going fine for the last month, the discus are thriving, eating tons, everyone else was active with no signs of stress.
Until today - I did a 50% water change last night with water that was heated, treated for chlorine and aerated for 24 hours - I do 20% once a week and 50% every second week and have since I added the discus with no problems. The only thing that was different was that last night I added a slightly lower than normal dose of liquid carbon for the plants - the first time I've done so since the OD a few months ago.
Came in this morning and all four boesemani's rainbows, both dwarf blue rainbows and the dwarf gourami were dead.
The tank is aerated with a pretty powerful airstone and has a low, but decent amount of current. Every one of the fish that died was completely healthy when I left last night and the gourami and dwarf rainbows have been in the tank since I set it up.
The discus are fine, in fact they were happily devouring the bodies when I got here. The cardinal tetras are fine - and they are supposed to be the fickle easy to kill fish.
Has any body ever had a massive fish death after adding a lower than normal dose of liquid carbon? does it acidify the water (which would explain the discus surviving)? Ammonia = 0, nitrites = 0, nitrates = 0, I don't know the exact pH or hardness, but Calgary water is typically slightly acidic and on the hard side.
Sorry for the novel, but any help would be hugely appreciated