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Year-round water temp control (Partial Re-use system)

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Myself and a buddy of mine are currently starting the design process for a small scale/Pilot project, partial re-use system for raising Tilapia. 

He recently purchased a farm in a tropical region of Colombia which has a year-round stream/waterfall flowing right through the property. We have been discussing the idea of trying to take advantage of the stream by building a pilot project for an 80% RAS//20% flow-through to get a better idea of the economics behind it before entertaining the idea of anything commercial. 

The one area where were getting stuck is coming up with the most efficient way to control the water temperature in the system. 

The temperature of the incoming flow is around 70-71 degrees F, (We've been checking it multiple days at different points throughout the day). Considering were located right on the Equator, There isnt any seasonal changes in ambient temperature...Were looking at year-round avg daily highs of 82-85 degrees, avg nightly lows of 61-63, (overall avg temperatures right around 71 degrees throughout the year. 

We are trying to figure out the most efficient way possible to keep the water temps right around 75-77 degrees consistently throughout the day.  I've spent all week trying to familiarize myself with the Thermal aspect of water temp control but I just keep going around in circles with all of the available ways to heat the water. 

Were set on starting with 3 tanks, 2000 Gallons each with a replacement flow-rate of around 7 gpm, and the recirculating rate of 27gpm. The tanks will be installed outdoors but we are not opposed to putting up a greenhouse/plastic coverhouse over the tanks but I cant imagine that alone making much of a difference considering there wouldnt be any insulation. 

Immersion heaters work, but I have to believe there is a more economical way to taking advantage of the climate here. Granted we will have to regulate the temperature through-out the year...we are realistically only a few degrees away from Optimum growing conditions. We realize we will not only have to maintain the temperatures in the tanks, but also attempt to raise the temp of the incoming replacement flow a few degrees to try and keep things consistent.  

So far i've looked into In-line heaters for increasing the replacement flow temps, immersion heaters for keeping the growout tank temperatures within range, Heat exchangers/pumps to recover some of the heat from the 20% that is exiting the system. But realistically, I cant figure out which might be overkill, in-efficient, or theoretically incorrect lol Also, would incorporating a Solar heating system be complementary? How do I even begin the process of sizing it considering our production parameters? its usually around 75 degrees by 9am, 85 degrees by noon, and 75 by 5pm, so there's about 8 hours throughout the day where ambient temperatures are just perfect. 

Sorry If im being redundant, thermal energy is just not my cup of tea. But if anyone can point me in the right direction, any insight would be appreciated. 

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