At a loss with an aquarium disease

[MitchM]

Do you have a substrate in these aquariums?
If so, what is it and how often is it cleaned?
Any planted areas that have never been cleaned, driftwood or other ornaments that have never been moved or cleaned under?

Old tank syndrome usually refers to a buildup of detritus in the substrate, or in the case of salt water aquariums, an additional buildup of heavy metals in coral skeletons and live rock.
The coral skeletons will stop contributing to your KH once the PH reaches 7.

Sorry for your losses. I have a lot of cory babies right now as well.
It would be heartbreaking to lose them.

 

[Shdwdrgn]

The substrate is white quartz sand (pool filter sand). She uses the python to clean up the sand at every water change, and a couple times a year she will get in there and move things around to clean up better under the logs and plants. We also have some golden dojo's in the tank who love digging through the sand, so it gets sifted around quite a bit.

Honestly if the pH stayed near 7.0 I'd be happy. Whatever happened in these tanks drove the pH way down which caused all the other problems. We're getting some crushed coral to put inside the filters for the different tanks, figure that will have more affect than just having the large lumps sitting in the bottom of the tanks.

 

[Shdwdrgn]

So I wanted to let everyone know that the aquarium is finally recovering. The ammonia had dropped to zero yesterday, nitrate/nitrite is pretty much zero, and the pH has been holding around 7.4. We lost the giant danios, but all of the larger fish have recovered and are doing great. Today we did another water change and added the crushed coral to one of the filters. Just have to keep an eye on it and make sure it doesn't crash again.

We also tore down and cleaned out the tank with the african cichlids. Found quite a few more bodies trapped under the rocks. The tank has been completely cleaned and we set it back up with part of the original lace rock, plus a really large chunk of driftwood and some plants. We're going to start over with kribenzas and several other fish that my wife has been looking at, who will also enjoy the rock caves. Its still really frustrating that we lost so many fish all at once, but now we know the signs and we can stop it when it starts to happen to other tanks.

 

[bettasngoldfish]

How are things going?

 

[Shdwdrgn]

Much better... we managed to keep the 120 gallon fairly stable for a few days while it cycled, and it is running on its own again. The 115 and the 20 have also been cycled and we have a couple new fish in each to test if the tanks are safe again (because water tests don't always tell the whole story). So far so good, although we're seeing signs of distress in other tanks as well. This can't be a coincidence, the city must have changed something in their filtration, however we have plenty of crushed coral to go in the filters of all the tanks, so we're slowly adding to each when we clean the filters.