No idea if there's a way to calculate waste, but its generally known that pleco's, mollies and platies have larger bioloads than most other fish... They poop a lot. If you keep them, it won't take you long to see they poop a lot; very rarely do I look in my tank and not see my platy pooping lol. When it comes to what size tank a fish needs, it's activity level is also taken into consideration. Take a Molly for example; mollies are active fish and need swimming room. Putting a Molly in a 10g would be like keeping a dog, like a German Shepard for example, that needs room to run around in a pen 24//7. Simple research will tell you if a fish is a more active fish or not.
Hehe, they are funny how it dangles behind them.
Ok, imagine a rating system that incorporated how active, how much they eat ,how heavy their waste was (and therefore how slow it would degrade) and all those things according to their adult size. Mixed it with what level of different toxins they can tolerate and came to a basic letter color and number rating eg redA1 indicating highly active heavy poopers that grew quite big and were very sensitive to nitrates say 5ppm threshold . And then had tanks plants and filters show how they affect the tanks environment to achieve suitable conditions.
That'd be an awesome book.
Is there already a book like that?