Does anyone happen to know how aqadvisor calculates its WC schedule?

[RennjiDK]

Is there a certain setpoint of "X bioload generates 10ppm NO3/PO4 weekly" or something to that effect? I assume its calculations are for a sterile tank with no nutrient export?

[Fish Folk]

According to the webpage, the water change schedule is calculated based on the total bio load of the selected species.

There are many factors to consider. Water changes are helpful for elimination of excess nitrates, reconstituting buffer, rebalancing pH, diluting chemical buildup, etc.

Some aquarists have particularly unhelpful source water parameters, making water changes a challenge. For others, it is advisable to change 30% every two weeks on normal aquarium setups, and 30% once a week for more advanced scenarios -- e.g. delicate species, or certain breeding or fry grow-out tanks.

If you can get a Python hose (or similar) to attach to your home faucet to change water quickly, the process is not too much of a chore.

Now, there are methods of keeping fish that can rely on a different way to achieve balance. With a thick substrate bed, and a heavily planted aquarium, tanks can achieve a balance that requires only the occasional "topping off." This has been advocated by some well-known, experienced aquarists in recent years online.

I advise learning exactly what is going on in your aquarium as best as you can before going down the "no water changes" rabbit hole. 

[nabokovfan87]

My gut tells me it's tied to the overall max size of the fish.

[Ken Burke]

I don’t really know how they do it. But I kinda like the way Cory explained in one of his videos. The short of it is to test your water weekly. When the nitrates get too high, do a water change. Repeat. Over time, you learn how much and how often to do changes.  if your bio load is low and you have a heavily planted tank, the intervals can be rather long. If your bio load is high, like when I bred BNPs in a 20 gal,  water changes every couple days is a very real possibility. 
 

I think this is the video: