Help with plumbing. Pressure relief valve needed?

I finished putting together an auto refill system for the sump of my new 800 gallon aquarium and had a couple questions.
First let me just breakdown what I am trying to do.

I had a plumber tee off the hot and cold water lines and solder a ball valve on each.

From there I added a check valve on both the hot and cold lines and a thermostatic mixing valve downstream to regulate the temperature of the water.

After the mixing valve I placed a pressure reducing valve (set to 50psi) that goes into a whole house 20" sediment and carbon filter.

I then reduced the line down to 1/4" high pressure poly tubing that runs through two additional granular activated carbon filters. The line then goes into a float valve in the sump that opens when the water level drops to a certain point. (An automated refill system for water changes)

I began it testing last night and everything worked flawlessly when the float valve was open adding water to the aquarium sump. The water flow rate is around .5-1gph.

When the water level rose enough to close the float valve, after about an hour or two minor leaks began developing at the two inlets of the thermostatic mixing valve from what I believe to be an excessive amount of pressure. I shut down the hot and cold supplies and vented the excess pressure by pressing the air purge button on the top of the 20" whole house filter housing.

The float valve will only open when the water level drops from evaporation and when I perform bi weekly water changes. So it would open briefly for 2 times per day to replace evaporated water and 2 times per week for around 2 hours to refill after I do the water changes.

Is there a major flaw in the plumbing layout anyone can point out to me?

Would a pressure relief valve be a potential solution to the excess pressure build up when the float valve is closed?

Any help would be greatly appreciated.

Thanks,
Reed

your system ahead of the mixing valve should NEVER be able to generate enough pressure to cause the leaks. In addition, the pressure after the regulator has NOTHING to do with the pressure ahead of it. If the regulator is working properly, there is no "excessive buildup" when the float valve is closed. Your diagnosis is faulty.

Thank you hj for the reply. It is very much appreciated. I wanted to make sure that the plumbing layout did not have a glaring flaw.

So if its not pressure related it could be as simple as the rubber o ring not being seated correctly or sealing properly on the hot/cold inlet of the mixing valve. This evening after work I will take out the valve and add a very thin amount of silicone o ring lubricant to the two rubber o rings on the hot and cold inlet connections. To see if that helps with the slow leak.