Help me decide how to proceed.

[Fishyzwim]

So i have several tanks. One tank though i call the death zone. I have had it up for nearly a year. One at a time fish die. I think everything is healthy add fish, die. They take a long time to die. Like a month sometimes 2 a couple even lasted 6 months but they die. Yes, i do test regularly and that's never been an issue. I keep my nitrates under 5 and everything else at 0. Ph is a little high but it is in all my tanks and this is the only one with issues. I've taken fish in to the LFS. I've done tons of research. Whatever it is doesn't fit squarely into any category but closest it resembles is parasites but that conclusion is only because 2 of the 2 dozen+ fish showed symptoms long enough before death to give any clues. I've now taken out the last two remaining fish (they look healthy but so did most the others) and put in a smaller tank. For several reasons i am not going to tear this tank down all the way. Ain't happening. It's a 75 gallon tank. There is java moss and snails left . The snails i will gladly see die. But even they as much as they are breeding there's not as many as i would expect. They grow to adulthood at a much slower rate than any other tank. So how do i treat this. Before adding fish back. I was thinking crank the heat to 90 for a week. Or if thinking its necessary poor bleach in, let it run through everything.

P.S. like a dummy i used that tank to seed a sponge filter for another tank, i wasn't thinking because at that time i thought we fixed the issue. But those fish are fine. It's a fry tank. No deaths

 

[Colin_T]

What symptoms do the fish have in the death tank?
What fish have you had in there?
Are you getting the fish from the same shop?
How long have the fish been in the shop tanks for?
When does the shop do water changes and when do you get the fish?
What colour is the gravel in the death tank?
How often do you do water changes on the tank and how much do you change?
What sort of filter is on the tank?
How often and how do you clean the filter?
Can we get some pictures of the tank and fish?

 

[Fishyzwim]

Guppies, danios and shrimp. Different sources. Some of which were added from my other tanks and were fine before going in that one including 2 month old guppies that immediately stopped growing but no other symptoms until death. Most just hid a few hours before death. The two that showed signs got skinny. The gravel is tan color river rock on sand. I have a HOB filter plus two large sponge filters (same exact filtration setup i have on a 100 gallon). 25% water change weekly more if there's been a death. Since i treat all tanks the same and this is the only one with issues im pretty darn certain its an illness of some sort and I'm really over chasing that horse. I just want to make sure i don't infect new fish and what needs to be done to make sure that whatever it is, is gone.

 

[itiwhetu]

You say the pH is a little high, how high?

 

[Colin_T]

Picture of the tank?
Pictures of any fish that died?

It could be poisoning from something in the aquarium. I have seen tanks that look perfect and everything was bought from a pet shop and fish would just die for no apparent reason. It turned out to be bad gravel, leaching ornaments and some types of driftwood.

Having sand and gravel doesn't work well because the sand ends up falling through the gaps in the gravel. It can trap gunk and cause anaerobic pockets.

Guppies can have nasty external bacterial and protozoan infections when you get them from a shop. But they don't normally affect shrimp, and the sick fish usually has symptoms like white areas on the body. Plus if the fish came from your other tanks, then it's unlikely to be a disease.

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I would chuck the substrate and remove ornaments and flush the tank out. Then start again without any ornaments. Add a couple of cheap fish and see how they go. If they are fine for a month, then add one ornament and monitor for a few weeks. If they are fine, add another ornament and monitor for a week. Continue adding one item at a time and monitor for 1-2 weeks. If you add something and the fish start acting funny, that item is probably the culprit.

 

[Fishyzwim]

Here's a pic of the tank. It's low on water and needs to be cleaned right now but it hasn't had fish in it in a week so i haven't worried about it. Every one of my tanks is sand bottom gravel top. Some with that river rock and some with a smaller gravel. I have that exact skull in another tank (bought two from same place) and pieces of that same rock things in another tank. This is specifically why i added fish from my other tanks into here. I knew they were fine with the water parameters, they didn't have negative reactions to anything in the tank because there were duplicates or pieces off the same decor in there's, they were used to the environment, they were used to the amount of air flow, they were used to the same water change regimen, the same water treatments, the same foods, same temperature, the same plants, the same lighting schedule. There's not a single thing in that tank they weren't exposed to for a long time before going in it. And on top of that I'm pretty darn certain which fish started the death cycle.

 

[Colin_T]

And on top of that I'm pretty darn certain which fish started the death cycle.

Did you add a new fish to the tank before they started dying?
If yes, what symptoms did it have when it died?

Why do you have sand under gravel?

 

[Fishyzwim]

The first fish that died was one of the two that got skinny. It didn't happen right away but he was the only fish added after the initial ones which was doing fine. I like the sand for the plants but hated it kicking up when i cleaned and i felt i couldn't get everything out and when planting new stuff they liked to float out. So now i put a thin layer of gravel on top. I've tried just gravel and just sand and i really like this combo. Plants grow great, easy to clean and like with the moss and stuff i just pick out a few rocks attach it and it instantly blends lol. The only downside is if for any reason i need to tear down a tank or i want to change the landscape it makes it an absolute nightmare. Once that substrate is set IT STAYS!!! i have an unintentional valley in one tank, stuck with it, just decorated the valley.