Hi all. So many questions at the moment. I separate holding females when I see they are holding, sometimes they sit there for two weeks. How often should I do water changes for them and how much? I don't feed them as they won't eat anyway. Thanks
On its face, this seems like a really simple question, and DJRansome's answer is sufficient. Change the water - easy enough, and please do so. Lots of research has shown that high nitrates (under 80ppm) are not generally acutely dangerous to our fish EXCEPT at the egg and fry stages. Multiple studies have shown that high nitrates have high impact on these fishes especially, so keep them low for a holding female.
But then I wondered what I would do. I don't want to keep fry, so the above does not factor in with me. I would test the water and change it when it needed it.
But then i got to wondering how long that would take. I would assume that with one holding Mbuna (assumption, as it's what I keep), and in a 10 gallon tank (?) you could go the duration of the hold with no water changes, if you're not feeding.
I would also go out there and say that depending on the mass of the Mbuna, you could keep any in a tank that long without water changes, even feeding - If you're feeding correctly.
Ammonia and other forms of nitrogen come from proteins breaking down. In a fish tank, the proteins that are breaking down are inside of the fish. They are NOT influenced by the amount of food the fish eats in a meaningful way. If there is excess food breaking down in the tank, then that absolutely would contribute to higher nitrogen from feeding. So, like DJRansome says, the fish is still making ammonia, you still need to do water changes. I would be incredibly interested to see if a male of similar size would produce the same amount of nitrate as the holding female over the same timeframe. This would, of course, depend on the metabolic weight of the fish and not just the absolute weight.
For me, this is at best an intellectual curiosity, as I would be testing the water to make sure. I try to take an evidence based approach.