One way to cheaply cool slightly is to bury some water line. Use the ground for a heat sink. I buried some tubing when I ran power trench. Just in case but don't need it yet. Not sure what your ground temp. is. Trout probably need water cooler yet.
What is your climate and what season will you be trying to cool through? What type of system?
Having a fish tank and plumbing in the ground if your ground temp is cool enough will help some but if it is really hot out and you are running flood and drain gravel you might not see much benefit.
They make chillers specifically for cooling fish tank water, chances are any other method is going to be less efficient and only worth it if the materials are free and the fridge already kinda ruined. Chilling water isn't cheap.
Trout can survive water warmer than 60 F but I think 55-60 F is like prime growing temperature for them. I believe the danger zone for trout comes as the water temperatures get over 70 F
The subject of cooling towers will probably be a growing concern as our AP industry advances.
While researching the design of cooling towers, I was made aware of the dangers of "Legionella". Yes the dangerous bacteria cause of Legionnaires Disease must be watched for when air/water handling equipment and food production meet.
"If the volume
of decomposing debris exceeds the chemical dosage's ability to provide control,
the cooling tower will silently grow dangerous even while chemical dosing
There is pleanty of info available to keep it safe and simple. Awareness is the key.
Ice blocks are the cheapest way to cool large volumes of water. I would place the ice in another container and run water through it before adding to the main tank so there is less temperature shock.
In dry climates you can experience significant cooling simply by allowing some extra evaporation.
Evaporating precious water? Tsk tsk.
Well, ya gotta make your choices, a little evaporation or running a chiller, I think in a dry climate I would choose the evaporation over a chiller. Here, I just grow fish appropriate to the climate.