I have an interesting "problem" in my shrimp tanks at the moment since having switched to RO water and the tanks having nicely matured and settled. The problem is this:
I can't do water changes because my water is already too clean!
To be more specific, I want to do water changes to vacuum the tank, suck out leftover food, suck out (nice green shrimp food type) algae that I clean from the front glass, etc. But my TDS remains really low, so I don't want to do water changes to lower it further. My KH and GH are spot on so I don't want to increase TDS with additional minerals in the w/c water, I just want the TDS to rise a bit through normal addition of ferts, food, leaves and driftwood breakdown, etc. But it really doesn't budge, even with full on Seachem dosing at 250% N, 200% P and 100% K plus Flourish and Trace (based on Seachen recommended dosing), adding liquid "leaves and bark", having big bits of driftwood, lots of leaves, etc, etc.
The only plants in these tanks are lots of Christmas moss and salvinia - I believe it is salvinia minima. Even with the fert dosing, I rarely get a reading for nitrate and never any sign of phosphate in any of the tanks at the end of the week. The reason, I think, is the salvinia which seems to suck up everything from the water column!
My plan was for weekly 5-10% water changes, but even after 2 weeks that seems unnecessary at the moment, so I'm now looking at a minimal water change about every 3 weeks. So, what I am trying to say is.... get some salvinia if you're looking to extend time between water changes!
(at least from an excess ferts point of view). The roots of the variety I have (Minima?) extend about an inch down into the water so not at all intrusive. I love the lush green colour, shrimp like to hang from it and eat it as well as from it, it grows super fast but is easy to remove and manage, and creates nice areas of shade in the tank below to help prevent algae. It is somewhat overgrown in the photo below - normally I keep it outside the black floating ring - but the tank is full of shrimplets at the moment so don't want to risk removing anything in which they might be hiding.