Opps forgot to say, if it was mine I would do huge water changes till it's fully fresh. If something damaging was in the water and it sounds likely as your fish has improved ... 10% leaves 90% of the bad stuff still there. I know people say you shouldn't for disturbing the denitrifying bacteria but I have done 100% water changes lots of times (not cleaning the filter out) over the years in emergencies in one go without it being a problem.

Sorry to hear about your fish, are you sure your Ammonia was zero and you are doing the test right, when you got back? I ask because a number of decaying fish in the tank should have increased the Ammonia to quite a high level, then obviously you would see the corresponding Nitrite then Nitrate high. Do you check Ph? Did you add any new decor or any new fish just before leaving?

Last and it is unlikely, do you disturb the substrate whenever you do a water change? You should if you can to stop noxious gasses building up. It the build up then bubble through the water causing very unhappy fish. It seems unlikely in a tank that is fairly new though.

It will not be lack of food, if one dies the others would eat it and not die as they had food

Also, I agree more big changes, should be fine, I too have done them many times and not had an issue.

Good call. My own water has absolutely zero kH and gH and I have to buffer it or the pH will crash in no time at all.

I was also thinking about the testing, even if the filter was coping with the big waste increase from the deads you would see it reflected in your nitrate level- are you using drop tests or the dipstick type tests?

Thanks for the advice everyone.

I spoke to one of the guys in the aquatics centre and he also assured me the fish would not have starved in just 5 days (they had clearly been dead at least a couple of days when I got back) and that even if some had, the catfish certainly would have eaten them.

He thought it sounds like something crashed early in the week which messed up the water - particularly as the survivior was the betta who can surface breathe - but corrected itself by the time I returned; maybe a power cut or something.

I'm only using dipstick tests which I know are not the most accurate but they've been working fine until now (as in, they did register rises and falls in ammonia / nitrite / nitrate as I added the various fish so I know they work). The nitrate had increased from virtually nothing when I left to the first level of "needs water change" when I returned so it had increased - not sure if this is the expected amount obviously.

I can't be 100% certain of anything but I would put money on Dali NOT attacking my other fish. None were species that could be mistaken for another male betta, and the whole lot had been together for about 3 weeks (the catfish longer) with no issues. He's so placid I really doubt he would so much as give the others a dirty look.

For the time being I'm just keeping up with the daily partial water changes and testing. I'm not planning on adding any more fish until I'm certain its all new water in there just in case.

Forgot to add - my pH is still reading the same (can't remember what the figure is but fairly high).

__________________
"For all its sham, drudgery and broken dreams it is still a beautiful world"

It makes me wonder if there was anything in the water that you used to perform your water changes before you went away. Before I had an RO unit I used to filter my water through a polyfilter for 48 hours before I used it
Polyfilter

__________________

Ok, so I've been doing a lot of water changes (10-20% daily) and testing the water every couple of days.
I'm now noticing a consistently high level of nitrates this week, with todays being the highest (about 100mg/l according to the dipstick) despite the fact I did my biggest water change last night.

Just tested a glass of plain old tap water and got the same reading so I'm assuming this is the cause of this particular problem. I'm wondering if it could also be the reason for the deaths as I did extra water changes before going away?

Any ideas what's the best way to deal with this? Would it be worth me buying in RO water for water changes? Or would I be ok just addig some live plants to use up the nitrates (only have plastic ones currently)?

Hawksport - how does the polyfilter work exactly (physically I mean - what do you do with it?) Would this remove nitrates? Being stuck in flat with two cats leaving a bucket of water around for 2 days would be a disaster waiting to happen!

__________________
"For all its sham, drudgery and broken dreams it is still a beautiful world"

Not sure of the truths in this, but I was told most water treatments are put in to the mains supply at the weekend, not sure if this might be the reason behind the hight Nitrates. I also thought the UK legal maximum for water was 50 ppm

I normally buy RO water, as it isn't too expensive and I only have a 55 litre tank. It may be worth you buying it until things settle down. I also get the occasional water test done at my aquarium shop - they may be able to advise you on what might be going wrong and what to add to the water to get back to the ideal balance. My local shop will test the common 6 levels, again not expensive.

Good luck.

I agree with what has already been said about your test readings. It's really strange not see any ammonia or nitrite reading in a tank that has suffered such a catastrophic loss.

I would get rid of the strip tests as soon as possible and get some proper tests. The strips are notoriously bad. I've had some in the past hoping that they would be a quick win solution to normal tests but when I compared the results with other tests they were rubbish.

Also, might be an idea to check with your electricity company, or your neighbours to see if there were any power outages while you were away.

__________________