1 liter of water frozen to 14F (doable with most freezers) will drop 10G of water about 5 degrees, 20G of water about 2.5 degrees, 40G of water about 1.25 degrees, etc. However this is a one time process since as soon as the water is back at tank temp no more cooling will happen, you can then repeat as necessary, also this doesn't take into account the heat from the room entering the tank, or the heat added by equipment (that 50W return pump is adding the same heat to your tank as a 50W heater), useful for emergency tank cool downs as necessary if you have a lot of frozen water bottles on hand but really only moderately useful for smaller tanks, a 100G tank will only drop 1 degree with a frozen 2 liter in it and again that 1 degree is the energy put into the frozen water there's still the ambient energy going into the tank.
Bottom line, put fans across the surface, refill evaporated water with RO, and turn down any lights (yes even LEDs).