In an aquaponic system your plants are ultimately eating fish food, the fish are just the middle man. Therefore, its helpful to know how much your fish weigh to calculate how much they are eating and how much plant growth they can support.
The key ratio of an aquaponics system is fish food to plant grow space. Leafy green plants will require between about 20 and 50 grams of fish food per day per square meter of grow space (assuming a fully utilized space). Fruiting plants like tomatoes and cucumbers will require a higher ratio between 50–80 grams per meter squared per day. (Somerville, et al 2014)
Once you figure out how much fish food your plants require, you work backwards to figure out how much fish you need. And this is based on a standard fish diet of about 2% of their body weight per day.
If you are adding too much fish food then your water quality can decrease as surplus nutrients build in the system and there’s not enough plants to filter the water. Conversely, not enough fish food and your plants won’t have enough nutrition. So know your fish weight!