I have a 10g breeder tank with low light, sponge filter, marbles on bottom. I used this tank to breed tiger barbs which was way to easy, got bored, sold them all to lfs. Lfs told me they would really like another local neon tetra breeder. I conditioned some, picked out a pair, put them in breeder tank, they got sick. Decided to try gold pristellas since they spawned in the planted tank and 2 survive to this day. They got fin rot. Perfectly clear water, lots of water changes, vac between marbles, kept the tank cycled when not in use, 0 a, 0 nitrite, <20 nitrate. Anyway, I keep reading how poor water conditions are usually to blame for these problems. I reach in and squeeze my perfectly clean looking sponge filter and OMG! a black cloud of nasty instantly rises from it. Oops, I guess I should've known better. But...clear water with good test results, I don't know?

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Odd mix of hobbies- fw aquarium keeping, muscle cars, general aviation(private pilot), firearms, home improvement projects, architectural design, the US constitution and last but first-my sons.

I have yet to see a test kit for bacterial life, or parasites. Sometimes we focus too much on the tests, and not enough on the dynamic systems good test results foster. Just because the tank is 'clean' doesn't mean it's going to grow the life forms WE want... it just means it's a decent environment for something (possibly fish) to live in.
The test kits are just chemistry - interesting, useful and limited. I learned that the hard way too. ;-(

Nail on the head, right there.

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Odd mix of hobbies- fw aquarium keeping, muscle cars, general aviation(private pilot), firearms, home improvement projects, architectural design, the US constitution and last but first-my sons.

Don't believe that just because you squeezed the sponge and stuff comes out of it, doesn't mean much. There are many particulates in the water and they stay on the sponge when the water is pulled through.

Meaning??? The filter is probably still contaminated? Understood.
Meaning??? The water column has everything that the sponge has but in less concentration? Understood.
If not, then you lost me...easy to do

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Odd mix of hobbies- fw aquarium keeping, muscle cars, general aviation(private pilot), firearms, home improvement projects, architectural design, the US constitution and last but first-my sons.

No, meaning it is normal. Any tank would do the same.