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Why do we change water?

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Post Mon Sep 28, 2020 9:19 am
eraserbones Level 5 Member
Level 5 Member

Posts: 288
Location: Uptown, Minneapolis, MN
This is a question for Willie but I'm sure others will have thoughts :D

My first answer to why I do water changes is: nitrates. That means that in theory I should be able to test my water for nitrates and see whether or not a given water change is necessary (or, more realistically, whether the current water change routine is too slow or too fast).

Is that right, though? Are nitrates really the only build-up that we need to remove, or is nitrate level just a proxy for other things? If I have a healthy growth of salvinia gobbling up nitrates does that mean I don't need to do water changes? My instinct is to say 'no, do them anyway' but I'm not actually sure why.

Willie, I know you change 100% of your water every day, but surely you aren't getting a detectable amount of nitrate build-up overnight. I'm sure that experience shows that it makes your discus happier, but do you know what in particular is making them happier? Does a tank with daily 100% water changes even /have/ a nitrogen cycle or are you removing all the precursors before anything happens at all?

Post Mon Sep 28, 2020 10:50 am
willie Level 9 Member
Level 9 Member

Posts: 1628
Location: Minneapolis
Absolutely correct, I'm not changing 100% daily in the discus and angelfish tanks to reduce nitrates. These tanks are barebottom and have relatively low bioloads. I doubt if my nitrates ever exceed 10 ppm and most aquarium fish can handle 100 ppm with no problem.

Discus, however, have very little tolerance for microorganisms in the water. They live in low pH (which significantly reduces bacterial growth) in the biggest flow system in the world. There's so little microorganism in the water for fry to feed that they have had to evolve a novel approach to produce skin secretions to raise their spawns. To fully exploit the genetic potential of these fish, I make huge water changes and wipe surfaces which get slimy. Except for certain unique genetics which limits growth, adult discus should grow to 7 inches.

Angelfish live in the same biotope as discus. Most people are not aware that they also have a rudimentary slime production system also, implying some sensitivity to microorganisms. One trait I prize in angelfish is long, thin and completely intact dorsal and anal fins. It's hard to find fish with intact finnage and most people talk vaguely about ammonia spikes burning them off. I suspect that it's actually microbial activity. Here's a picture of koi angelfish. Not a great picture, but I ascribe these long, thin dorsals and anals to large, daily water changes. Notice that the pigmentation runs the length of the dorsal, so there's never been any damage to the tips.

IMG_1838(MFFLjcBtlRt).JPG
IMG_1838(MFFLjcBtlRt).JPG (34.93 KiB) Viewed 288 times

Other species in the Amazon are perfectly fine with having fry scrounge for themselves in the leaf litter, so I don't think this sensitivity is common to other species.

As for cycling, I only run sponge filters and they're fully cycled. They get squeezed biweekly and are actually slimy to the touch. All that cleaning means my sponges start to droop after 10 - 12 months. But if anyone needs a cycled sponge, I offer them free of charge with 48 hours notice.

Also there's the OCD... :?

Post Mon Sep 28, 2020 11:07 am
eraserbones Level 5 Member
Level 5 Member

Posts: 288
Location: Uptown, Minneapolis, MN
willie wrote:
I suspect that it's actually microbial activity.


Have you or other discus people experimented with UV filtration? That might address the microbes in the water column; it certainly works great vs. phytoplankton.

Post Mon Sep 28, 2020 12:43 pm
willie Level 9 Member
Level 9 Member

Posts: 1628
Location: Minneapolis
I haven't as most bacteria reside on surfaces.


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