I did a 20% water change yesterday and when I came home today 4 fish were dead. I have a 90l tank with 4 minnowa 1 pleco - which all died and 4 guppies and 4 tetra which are both fine. Did water check and the parameters are:

Ph 7.5 - 8
Ammonia 0
Nitrate- 10-20
Nitrite - 0

They were all fine ths morning. BTW I fed the pleco 1 algea wafer last night. It was the 2nd time I tried them- the 1st time the other fish ate it so i put it in when the lights went off last night.

Does anyone know what may have caused this of give mem any advice.

NONIE

How long have you had the fish,are they newly added,is temperature alright and also is the tank fully cyled ?

yes had the tank since January and the fish bout 3 wks later. temp is 24C the pleco was added Sat week and has been fine since we added him.

you could add another degree to the temperature I think but this wouldnt of caused its death to be honest. The Pleco may of bought something with him when he was introduced, did you quarantine the tank ? However it seems strange that all of the fish died overnight, it seems like something happened rather suddenly as opposed to a build up of something. Im guessing the fish were behaving as normal before they died, the only thing that seems to have been a possibility is the algae wafer may of been contaminated or something. Its a strange one alright, I think someone like Darragh or Goldy may be able to offer more of a help on this. They have tons of experience.

Thanks Fishowner for your help. I didn't quarantine the pleco when i got it and the fish were fine this mornign. (I ahve a habit of checking them in the morning and when I get home from work!!). It does seem really strange that so many died over night.

Has anyone used algea wafers before and could this have been the problem.

I use algae wafers for my bronze cats who love them and they have all been fine to date. I even gave them a boiled pea tonight but its messy enough when they all go at it. I dont know if anyone else has had problems with algae wafers. Its seems rather mysterious, keep a close eye on the remaining fish, try and see if they have any problems by looking at them.(ie. are their scales normal, etc).

I checked all of them and they seem fine. They are looking helthy, feeding and swimming fine. I have just checked the water coming from the tap and that seems fine -no changes in it. It is a complete mystery as to what happened. I only lost 2 mollie a few weeks ago after feeding brine shrimp. I think I should give up the hobbie!!!

Firstly please dont give up. Everyone has problems at the start and during the course of fishkeeping, you learn one thing and then discover another that you need to learn!
Don't give up on the little buggers just yet. Do you dechlorinate the tap water that you add to the tank ? Also can you advise size of tank, filter, heater etc....as well as its inhabitants. Trying to get an overall idea here so we might be able to help you out.

I never declor the water from the tap because I was told from the lfs it is great for the fish. I have a 90l tank. fluval 2+ filter, 100w heater, airpump, 4 guppies, 4 tetra, (had 4 minnows and 1 pleco.) artificial plants and 1 ornament. As i said before I have had them 4 months or so and no probs!!

I do the same water change every week and rinse out one of the filter pads every now and then. I used a polyfilter pad the last few weeks but that has neer caused a problem!

Thanks for the support Fishowner,

Nonie

Ok first of all tapwater is NEVER great for a fish..your LFS is copmlety wrong there, chlorine kills fish so I would suggest you get some dechlorinate drops for the water you are adding. (i presume you bring the temp of the water up before adding it to the tank?)
Also when cleaning the filter pads, clean it in some water taken from the tank, dont clean it with tap water for the same reason as above,and you dont wanna kill the beneficial bacteria that you worked hard to get during you cycle.