Fish Load & Filtration Calculator
Stocking isn't set by gallons — it's set by how much ammonia your filter can process. Work it either direction: from the media you own, or from the fish you want.
Your biofilter media
Add up the credited surface area of every biological stage — bead filter, shower trays, moving bed. This is how a build gets specced.
The fish you want to carry
Size for the fish they will become, not the fish you bought. A 6" koi becomes a 24"+ koi, and its waste output goes up roughly sixty-fold on the way.
Media specification
These are deliberately conservative planning figures. Manufacturers often quote total surface area including micropores too small for bacteria to colonise; only protected, water-swept surface does the work. If you have a credited figure for your media, use Other and enter it.
Planning range is 0.2–1.0. Stay low unless you have measured your own system.
Feeding — this is what actually drives the load
This sizes the biological stage only. Solids handling, dissolved oxygen, flow rate and swimming space are separate limits that can bind before your biofilter does — a correctly sized filter still underperforms if DO is low or alkalinity is exhausted. Nitrifying bacteria take weeks to establish and slow sharply in cold water, so a system that computes fine on paper still needs to be brought up gradually and watched with a test kit.
