The freshwater aquarium, when indulging in reductionism, is a large glass box that you occasionally sprinkle some food into. This presents a problem, since the food isn't removed. To combat this, some bright spark placed a few fish into the box, and they obligingly ate the food and turned much of it into random movement and CO2.
Irritatingly, in order to continue living, the fish require some protein. When metabolized, the nitrogen on each amino acid diffuses out of the gills as ammonia where it promptly kills the fish. Fortunately, there are opportunistic environmental bacteria that make a living oxidizing this ammonia, using the energy to make more of themselves. During this process, they also need phosphate, another waste product to make DNA and yet more of that nitrogen for proteins of their own. Ultimately, the bacteria will stop growing once they bump into another limiting factor. This factor is usually a carbon source. Algae get around this by using CO2 and light energy, but the nitrogen oxidizers can't.
So, solve the problem by adding some sugar. Now you will have extra bacteria gradually building up in the tank and you're no closer to removing the nitrogen/phosphorus excesses from the food. So, the device.
On the back of the tank, we have the device. It pumps water out of the tank over a rotating plastic cylinder before allowing it to flow back. With added carbon, the rotating cylinder will be an attractive aerobic environment for the culture of the appropriate bacteria. The cylinder will have a spiral groove cut along it to add surface area and to guide a stylus-like follower. This follower will scrape off bacterial growth into a waste container as it progresses along the groove over a period of a week or so.
The grooves will be slightly too deep for the follower to completely clear, so that there's always a seed stock of bacteria able to repopulate the environment after the majority has been removed. The re-population is what consumes the excess nitrogen/phosphorous waste and the nutrients are exported into the waste collector which you rinse out every now and then. You re-set the stylus- follower and carry on with your life.
There, nutrient export for the freshwater crowd, take that you salty protein skimmers.-- bs0u0155, Sep 17 2020 [+]-- Voice, Sep 17 2020 <Choking and spluttering noises/>
// rotating plastic cylinder //
Fine for the prototype, but the production version will be crafted from precious metals and gemstones, shrley ? Otherwise it won't be nearly expensive enough to satisfy the typical obsessive tank-tender...-- 8th of 7, Sep 17 2020 //crafted from precious metals and gemstones//
No, still plastic, like all the other aquarium stuff, but we'll charge gemstone money. Esp. for the on going "fish, shrimp & pant safe carbon supplement".-- bs0u0155, Sep 17 2020 // pant safe carbon supplement //
Your strategem is revealed; you're going to charge so much money for this that you need locking metal boxes in your trousers to carry all the cash away ...-- 8th of 7, Sep 17 2020 //locking metal boxes//
carbon fiber for low weight and fewer inconvenient questions at airport metal detectors.-- bs0u0155, Sep 17 2020 Can't you just occasionally take some of the pisswater out and put fresh in? You could use it to water a potplant or tomatoes or something and save on babybio.
Either that or ...
//So, solve the problem by adding some sugar.//
...keep on adding sugar until you have a sort of fish jam. Now your fish are preserved!-- Loris, Sep 17 2020 When did you last see an aquarium indulge?-- pertinax, Sep 17 2020 random, halfbakery