Backyard Aquaponics
A place to discuss aquaponics

I have 2 Grow beds with 511 sq.f /each with 30cm of water and using for lettuce(DWC system).And i have 3 IBC fish tanks with 264gallons/each.

My questions is how much tilapia should i have for feed nutrients all of this system.
thx for advice.

So roughly 47.5 square meters of raft. According to UVI you need 60 to 100 gm feed/m2 of raft per day depending on the crop you are growing. You need to look at how much the fish will eat and how large you plan to grow them. This has some sample calculations - http://aquaculture.ca.uky.edu/publication/srac-454-recirculating-aquaculturetank-production-systems-aquaponics-integrating-fish

You can also look at the lakeway tilapia site and guesstimate it based on the daily feed rates by size of fish and the UVI numbers. If you're going to run a setup as large it might be good to invest in some education through someone like Ryan Chatterson - http://www.collegeofaquaponics.com/author/ryan/

UVI has 4 tanks with staggered fish production and harvest so that they always have sufficient nutrients coming in for the plants. You can probably do something similar with your three IBC's provided there is enough volume to have as many tilapia as you need to support that raft area.