It's unfortunate that my first post is under dire circumstances; I imagine that's not uncommon 
Over the last few days I've had a sequence of deaths in my aquarium, and I'm at a loss as to why - would appreciate help diagnosing this, and figuring out next steps.
Tank stats:
110 liter tank has been running for nine months. Was cycled slowly, fish were added gradually. Two weeks ago the residents consisted of:
8 zebra danios
17 harlequin rasboras
1 guppy
For the last few months parameters have been as follows every time I've tested the water (recently, every day, sometimes more than once):
pH 8.3
Ammonia 0
Nitrite 0
Nitrate 10-20 ppm (I've never been able to tell the difference with the API test kit).
Recent (last month or so) temperature: 28 - 29.5c, has gone up to 31 once or twice (we've had a series of heat waves).
Up to one month ago temperature between 26c and 28c.
Tank has some brown algae, some white fluff growing on driftwood, and (recently) a few white spots on back glass.
For the last two months or so have been doing a 10% water change every day or two
Symptoms and recent history:
11 days ago introduced 14 zebra danios two the tank. One died the next day - it had been weak and emaciated already in the bag. I hadn't noticed this when leaving the lfs. after a few days during which fish looked healthy and water parameters were normal (see below for exact numbers) five days ago I introduced two juvenile bristlenose plecos and 5 cardinal tetras. I acclimated the fish by floating bag in tank for 20 minutes, then adding some water from the tank, waiting another twenty minutes, and then transfering the fish to the tank with a net.
One of the BNs died within two hours, quite horribly - made multiple, frantic, runs towards the top of the aquarium, as if it was trying to jump out. The remaining fish looked healthy. However, all five cardinal tetras died between 24 and 48 hours after introduction. Deterioration was very rapid - from a healthy looking fish the colors would fade, and swimming would become dizzy - like a corkscrew as the fish swum through the water. Within a few hours the fish would die.
I was distraught, as I assumed I had made some error introducing the fish to tank, causing their deaths. However, every day since then I've woken up to find 5 or so dead fish in the tank, and possibly one or two sick ones. By now I've lost more than 50% of the tank population - the second BN, 13 rasboras, and a few danios. Miraculously the guppy is still alive, though it doesn't look well.
The main symptom I've noticed in the rasboras is this "corkscrew swimming", which lasts a few hours, until the fish settles at the bottom and dies.
The danios, for the most part, have become a little listless and hang out near the top of the aquarium (though I haven't noticed them gasping for air at the top).
Ancient history, in case it's relevant:
As mentioned, tank was started nine months ago. Fish were added slowly with no catastrophies for a while. At its peak the tank had:
20 harlequin rasboras
20 zebra danios
15 guppies
2 bristlenose plecos, one juvenile and one fullgrown
Started out doing 20%-30% water change per weak.
About four months ago we lost all the guppies within a month. The other fish for the most part seemed fine, and the water parameters were normal (as specified above). I chalked this up to weak guppy stock, so I waited a month to see that the tank was stable, and introduced 15 guppies from a different store. Everything looked fine for about a month, but then the two BNs died within a weak. I checked the water params and noticed that pH, ammonia, and nitrite were as above, but nitrate was at 80 ppm. I increased frequency of water changes to 20% twice a weak, which lowered the nitrate level. However, the guppies started dying off at a rate of one every few days, and now only one left.
After all but one guppy died, waited a month to see that the tank was stable. During this month the fish looked healthy, but brown algae started growing all over (presumably because no BNs to keep the aquarium clean). This brings me to two weeks ago, with the remaining story above...
Over the last few days I've had a sequence of deaths in my aquarium, and I'm at a loss as to why - would appreciate help diagnosing this, and figuring out next steps.
Tank stats:
110 liter tank has been running for nine months. Was cycled slowly, fish were added gradually. Two weeks ago the residents consisted of:
8 zebra danios
17 harlequin rasboras
1 guppy
For the last few months parameters have been as follows every time I've tested the water (recently, every day, sometimes more than once):
pH 8.3
Ammonia 0
Nitrite 0
Nitrate 10-20 ppm (I've never been able to tell the difference with the API test kit).
Recent (last month or so) temperature: 28 - 29.5c, has gone up to 31 once or twice (we've had a series of heat waves).
Up to one month ago temperature between 26c and 28c.
Tank has some brown algae, some white fluff growing on driftwood, and (recently) a few white spots on back glass.
For the last two months or so have been doing a 10% water change every day or two
Symptoms and recent history:
11 days ago introduced 14 zebra danios two the tank. One died the next day - it had been weak and emaciated already in the bag. I hadn't noticed this when leaving the lfs. after a few days during which fish looked healthy and water parameters were normal (see below for exact numbers) five days ago I introduced two juvenile bristlenose plecos and 5 cardinal tetras. I acclimated the fish by floating bag in tank for 20 minutes, then adding some water from the tank, waiting another twenty minutes, and then transfering the fish to the tank with a net.
One of the BNs died within two hours, quite horribly - made multiple, frantic, runs towards the top of the aquarium, as if it was trying to jump out. The remaining fish looked healthy. However, all five cardinal tetras died between 24 and 48 hours after introduction. Deterioration was very rapid - from a healthy looking fish the colors would fade, and swimming would become dizzy - like a corkscrew as the fish swum through the water. Within a few hours the fish would die.
I was distraught, as I assumed I had made some error introducing the fish to tank, causing their deaths. However, every day since then I've woken up to find 5 or so dead fish in the tank, and possibly one or two sick ones. By now I've lost more than 50% of the tank population - the second BN, 13 rasboras, and a few danios. Miraculously the guppy is still alive, though it doesn't look well.
The main symptom I've noticed in the rasboras is this "corkscrew swimming", which lasts a few hours, until the fish settles at the bottom and dies.
The danios, for the most part, have become a little listless and hang out near the top of the aquarium (though I haven't noticed them gasping for air at the top).
Ancient history, in case it's relevant:
As mentioned, tank was started nine months ago. Fish were added slowly with no catastrophies for a while. At its peak the tank had:
20 harlequin rasboras
20 zebra danios
15 guppies
2 bristlenose plecos, one juvenile and one fullgrown
Started out doing 20%-30% water change per weak.
About four months ago we lost all the guppies within a month. The other fish for the most part seemed fine, and the water parameters were normal (as specified above). I chalked this up to weak guppy stock, so I waited a month to see that the tank was stable, and introduced 15 guppies from a different store. Everything looked fine for about a month, but then the two BNs died within a weak. I checked the water params and noticed that pH, ammonia, and nitrite were as above, but nitrate was at 80 ppm. I increased frequency of water changes to 20% twice a weak, which lowered the nitrate level. However, the guppies started dying off at a rate of one every few days, and now only one left.
After all but one guppy died, waited a month to see that the tank was stable. During this month the fish looked healthy, but brown algae started growing all over (presumably because no BNs to keep the aquarium clean). This brings me to two weeks ago, with the remaining story above...
