The first thing is to figure out the surface area of your bio-media. 1/2" pea gravel is about 90 Ft2/Ft3. Scrubbie pads are about 400 Ft2/Ft3. When mature, a bio-filter can remove about .05 grams of ammonia per square foot (Ft2) per day. To calculate how much ammonia you can remove, multiply the total surface area times .05. That means 1 cubic foot of scrubbie pads can remove 400 X .05 = 20 grams of ammonia per day. 1 cubic foot of growbed made from pea gravel would remove 90 X .05 or 4.5 grams per day.

To figure the bio load from the fish you need to know how many pounds of fish you have and how active they are. A good estimate is about 5 grams of ammonia per 100# of fish per day. Over estimate your poundage as fish GROW!. Be sure you have more removal capability than you have supply. A safety factor of 2 should be considered minimum.

Next you need to flow the water to the bio-filter fast enough. Ammonia levels will build up until the amount flowing into the filter equals the amount supplied by the fish. I'll do the calculation in liters because it is easiest that way. You can always convert to gallons later if you want.

Suppose you want to limit the ammonia levels is less than .1 ppm (parts per million) That means you will have no more than .1 gram of ammonia in 1 million grams of water. 1 million grams of water is 1000 liters. If you want to remove 5 grams per day you must flow at least 50,000 liters per day through your filter. you get that by taking 5 grams of ammonia per day and dividing that by .1 grams of ammonia per 1000 liters of water. To get liters per minute divide that by 24 hour/day X 60 Minutes per hour or 1440. That gives you 37.7 liters per minute. This can be converted to 10 gallons per minute.

More fish means higher flow required.

The end product is nitrates and phosphates which must be removed by using plants or water changes. The thumb rule of having 2X the grow bed area as you have fish is based on having enough surface area to support the plants needed to absorb that. It is a VERY rough rule and depends on the plant and its needs. A heavy harvest will disrupt this, but not the bio-filtration of ammonia.