I need to draw on your experience.

I have a freshwater 55 gallon that has been set up a few years. 8 weeks ago my community fish all died over a 3 wee periods - barbs, tetras and platys. I took a water sample to my locally owned fish store. The owner tested my water and said all the values were in range. I practice good hygiene siphoning off 10-15 gallons every other week. I use city water pretreated with stress coat to replenish my water. My filter is an emporer with two bio wheels that has served me well. Artificial plants and no new decorations.

My only survivor is a blue crayfish that is a year old and is doing well. Three weeks ago I introduced one platy and it died within 12 hours. Again the water tested fine. This week I introduced another platy as a test and it was lying on the bottom the next am. The crayfish is still doing well.

The water is clear and odor free kept at a constant 76 degrees.

I have tried to address all the questions I would ask someone posing this problem to me. What am I missing?

Thanks

Were there any symptoms of illness? Your tank might have some kind of infection in it--maybe a bacteria.

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Creatures great and small, I love 'em all! But mainly the fish.

high/low ph? Maybe they need acclimated before going into the tank?

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

How long has the crayfish been in there?

I have heard that crayfish like to catch and eat fish sometimes. But are you using water conditioner to remove the chlorine? It could be that they went into shock frm not being acclimated properly?

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Frontosa Fanatic

Get a liquid test kit and post the results here. Also, the acclimating could be a good point, and crayfish do like eating fish (my LPS keeps theirs in a feeder guppy tank so they don't lose valuable fish)

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Lead me not to temptation...I can make it there myself just fine, thank you

Thanks for the responses.

Here are some answers to your questions

I use stress coat to pre treat water when doing water changes
After a few hours the new fish get lethargic and list to one side, then later settle on the bottom motionless.

The owner of the fish store that tested my water said the ph and other values were in range. I've never bothered with a test kit because I have never had water quality issues when I have done regular water changes and filter changes.

I will get a test kit.

what is the best kit that will identify the most likely toxins killing my fish?

Thanks

the API master test kit is what everyone reccomended for me. It tests PH, ammonia, nitrite and nitrate.

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

Get the test kit and test the water for yourself. Make sure to test for chlorine, ammonia, nitrite, nitrate and PH.
Also, when you put the fish in your tank are you also putting the water from the fish store in your tank. If so, don't do that. You are importing all of the water problems from your fish store to your own tank.
I hope you are buying your fish from a real fish store. Not a pet store that sells fish as a sideline, and certainly not from one of the big discount retailers.

If your fish are dying in 12 hours, there is something very wrong. If the fish are healthy when you put them in the tank, and your water, temp, and filtration are all right, then they should not die. Even ick and fungal diseases don't kill that fast.

You may have to drain and clean the tank. Please post your readings. Be very careful doing the tests. They are only accurate if done properly. And don't use paper test dip strips. Those are not accurate.

Fish are living animals. Treat them as carefully as you do any pet.

New to saltwater aquariums.
75 gallon FOWLR
1 yellow tang
2 yellowtail blue damsels
4 ocellaris clownfish

35 years keeping freshwater fish.

38 gallon freshwater
12 neon tetras
4 black skirt tetras
4 zebra danios
2 corys
1 chinese whiptail algae eater