Strap a graduated rod to the siphon for checking water usage by one growbed. Alternatively you can stick a piece of plastic ruler to the side of the fish tank, don't have one at the moment. Needed to do this to find out how many growbed this single fish tank can safely support.
Below is water level just before siphon start, that mean water is full inside the growbed. It stood at about the 140 mm mark.
At siphon stop, that when you hear the burp sound is at 95 mm marking.
Subtract 95 from 140 we get 45, so its 45mm or about 1.7 inches of water used for one of this growbed. This is the easy way to determine how many growbed we can use for a given tank.
Fish tank water height is about 17 inches. 2 growbed will take about 3.5 inches, and 4 growbed will take 7 inches. I can say 4 growbed is good for this tank. This water usage is at its maximum, with time we can reduce water height in growbed. On average four growbed in this setup will use less than 25% of total water, anything less than 30% is safe.
How many fish can be in there, well what you do is measure maximum water in tank and then get the minimum reading (fill all growbed) add the two figures then devide by two, we want the average.
If average is 80 gallon, I will put 80 full grown Tilapia but if you feel its too many just reduce it by 20%
In above example with four growbed it around (17 +10) /2 , so around 13.5 inches. This we need to multiply by tank area to get volume in cubic inches and then convert to gallon.
So around 50 x 30 x 13.5 = 20250 cubic inches equivalent to about 87.5 US gallon.
BTW, its recommended that you turn the water minimum 4 times in an hour so, if the tank capacity is 100 gallon you should total pump capacity of at least 400 Gallon or about 1500 liter per hour.
So having 2 growbed with 1500 to 2000 Liter per hour flow rate is sufficient to get this size tank cycle at minimum 4 times per hour.