All fish dead after water change - advice needed.

54and56

Hi all,

I've a large 600 litre tank hosting +/- 35 Malawi cichlids and 3 rusty plecos which I've successfully maintained for over 10 years.

The tank has 2 X JBL 1502 external filters and a Tunze 6065 Turbelle Wavemaker to ensure good filtration and plenty of water flow / oxygen exchange.

Yesterday I did a 66% water change and filter clean out (rinsing filters and biological media in expelled tank water etc) in the usual manner and last night the tank looked great with all the fish looking well and active but this morning I just discovered all the Cichlids are dead. The Pleco's are still alive.

I'm devastated but need to figure out what caused the wipe out so looking for a little perspective from other fishkeepers.

My normal monthly routine is to do a 50% to 66% water change and rinse out one of the two filters so each filter gets cycled every two months.

When filling the tank back up I use API Stress Coat (5ml per 40L of water so 50ml used yesterday) to make the tap water safe but yesterday I also used Tetra EasyBalance (2.5ml per 10L of water so 100ml used yesterday) which I've used before but don't use every time.

The only other thing (which on reflection I'm thinking is in fact the cause) is I removed the sliding glass panels from the top of the tank which a family member cleaned in a bath as they'd become fairly discoloured. I didn't pay attention to the glass cleaning as I was busy with filter etc so when the glass panels came back clean I just slotted them back in.

Turns out the glass panels were sprayed with a Kitchen Cleaner disinfectant (see ingredients photo below), scrubbed and rinsed.

I'm thinking residue from the kitchen cleaner must have mixed in with the tank water as I filled the tank right up to the glass panels so their full surface was in contact with the water.

Two things I'm looking for assistance with:-

1. Am I correct to think it was most likely residue from the disinfectant which will have poisoned all the fish or could it be that I overdosed the tank by using both Stress Coat and Easy Balance at the same time?
2. How do I ensure whatever killed the fish is expelled from the tank? I'm assuming I'll need to do several 90% water changes over a period of a week or two before I can try introducing new stock?

Thanks in advance for your input.

Feeling totally stupid and gutted so please go easy on me!!

scwazrh

1. Am I correct to think it was most likely residue from the disinfectant which will have poisoned all the fish or could it be that I overdosed the tank by using both Stress Coat and Easy Balance at the same time?

Would agree with you that’s the cause.Would also say 66% and a filter rinse is too much at one time but you’ve probably been doing that for years and they were able for it.The disinfectant would have a pushed the balance over the edge this time.

Not a nice morning for you.Very disheartening to lose the full stock at one time ,particularly Malawi.

How do I ensure whatever killed the fish is expelled from the tank? I'm assuming I'll need to do several 90% water changes over a period of a week or two before I can try introducing new stock?

Empty the tank , rinse and dry and repeat many times .Also I’d bin the substrate and scrub the rocks.No harm replacing filter media and then add fish as if it’s a new setup.

54and56

Thanks @scwazrh, I think you're right unfortunately.

Not sure what I'll do with the Pleco's whilst I'm rehabbing the tank and to be honest right now I'm thinking maybe now is a good time to quit altogether and just get rid of the tank as I don't have the motivation to essentially start again.

I'm probably just a bit emotional right now and in a day or two practicality will be restored!!