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Flourish Excel and Fish Deaths

I am wondering if anyone has seen a correlation between regular use of 
Flourish Excel at the recommended dosage and fish deaths? I am starting to 
wonder about this, and here's my story.

There are 6 fish tanks in my house. Two receive Flourish Excel at the 
specified dosage, 1 has canister CO2, and 3 get no supplementation at all. 
In the canister CO2 and non-supplemented tanks, fish tend to survive. There 
is the occasional fish death in those tanks - my 10 year old's, in 
particular, given he's sporadic about both feeding & water changes - but 
mostly, you put a fish in, and you enjoy it for years.

Not so in the two other tanks. My husband's tank is a nice, heavily planted 
10 gallon tank with amano shrimp (2), otocinclus (4), and tiger barbs 
(population varies). The shrimp and otos do just fine, but the tiger barbs 
only last 3-4 months before they begin headstanding and eventually losing 
their orientation altogther, getting stuck upside-down in the plants. He 
feeds his fish and adds Excel every night around 5:30, and the headstanding 
usually begins within a few hours. By morning, the fish are fine again. It's 
also noticeably worse after a water change, when he adds more Excel than the 
daily dose (per label instructions). But the fish are better by morning, 
until they're pretty far gone.

In my 29 gallon, moderately heavily planted tank, I don't see the 
headstanding behavior, but I have had lots and lots of fish die within weeks 
or months of being added to the tank. A partial list: endlers livebearers, 
blue-eyed rainbowfish (Pseudomugil furcata and gertrudae), cardinal tetras, 
ember tetras, phantom tetras, and gouramis (dwarf, dwarf red flame, and 
dwarf sparkling). In this tank also the bottom dwellers - zebra otocinclus, 
zebra loaches, and an ancistrus - seem to do just fine. When I started up 
the 72 gallon tank in June of '00, I took the cardinal tetras and forktail 
blue-eyes out of this tank and moved them into it, and they thrived - most 
are still alive today.

We have stopped using the Excel in both tanks, and the headstanding behavior 
in the tiger barbs has diminished. I don't know if the zebra danios and 
striped angelfish in the 29 gallon will live longer than previous fish. I 
know that Excel can be toxic at very high overdose levels (25 ml in the 29 
gallon killed several cardinals and had the suckermouth cats gasping at the 
surface). I am starting to wonder if it builds up in the tank over time and 
eventually becomes toxic? Both Excel tanks used to get 50% water changes 
weekly; recently they've dropped to every 2 weeks as the algae has gotten 
under control, but the fish deaths have been going on all along.

I want to stress here that this is my experience only. Your mileage may 
vary. I am not telling anyone to stop using Excel, which I have found to be 
beneficial to the plants.


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