Hydro (water) heating and cooling ?

Not sure if this is the correct place to ask this or not. It has been a long time since I have been on here. I am a farmer and not an engineer but, I am trying to set up a heating system for an Aquaponics system where we grow fish and use the waste to fertilize plants in water, which in turn cleans the ammonia out of the water before it flows back into the fish.

We are in the process of installing a hot water boiler to heat both the air inside the building (which is basically a plastic covered steel frame) and the water inside the fish and vegetable water. Now the water from the boiler cannot contact the water inside the fish or vegetable tanks and will be run through pex or stainless steel tubing.

We are using a heat exchanger of approximately 38,000 BTU to heat the water in the fish tanks to approximately 75 degrees Fahrenheit. The total fish tank volume is 4 tanks of approximately 250 gallon each or 1000 gallons. The total plant beds or tanks are another 4,000 gallons, although they are not as critical and can run cooler. Now, the water from the fish gets gravity fed from the fish tanks to the other bed tanks continuously. However, the heated heat loops inside the fish tanks will only be about 25 or 30 feet of 3/4 inch stainless steel heat lines each on Four separate zones so each can be regulated separately. The water cannot get over 80 degrees in the fish tanks or drop below 60 degrees Fahrenheit.My main concern and reason for this question, is the water coming from the heat exchanger to the fish tanks is going to be between 160 and 180 degrees Fahrenheit and needs to be cooled with a mixing valve to 120 or 125 degrees Fahrenheit before entering the fish tank. There are 2 reasons for this. The fish themselves and the microbial activity in the tanks cannot handle being in direct contact with anything warmer.

My question is: How much water reserve do I need to be able to run a mixing valve, mixing some of the return water into the 160 degree water to get it to 125 degree Fahrenheit so I do not cook the fish?

Things to note: 1.) Total furnace output is 400,000 BTUs to heat the structure and the water. 2.) We need approximately 300,000 of the total output of the furnace to keep the air heated on our coldest nights. 3.) I can run some piping through one or more of the vegetable bed tanks to cool the return water more if needed. Each of those 3 beds holds approximately 1200 gallons. 4.) The water for the fish in the fish tanks is flowing just about continuously through the system so being exchanged with the water in the beds. 5.) My main concern is that such short runs of pipe in the heated loops will not allow the water to cool enough to mix with the hot water in the feed side of the exchanger to keep the feed water under the 125 degree Fahrenheit limit. 6.) The heat loops are a closed system and cannot have cold water from a supply line added to cool the feed line water. So, I am forced to provide enough reserve to allow for enough cooler water.

I have tried to be as brief as possible and yet add enough of the details to give a complete picture of what we are trying to do. Please forgive the length of the post and if I have left out anything pertinent, please let me know and I will try to provide the information. I am hoping there are some Hydro engineers on here that love this kind of thing. There are just so many variables that I cannot begin to see a way to calculate the answer.

Thank you for any input you can give in advance.