The design and assembly of past modular cooling systems required that cooling modules with different fluids be separated, lowering the efficiency of the system. The present invention overcomes this problem by providing a modular cooling system with a plurality of cooling modules, one of which is a hydraulic oil cooler. The cooling modules are connected to a bottom tank which has a top plate with a pair of openings 64,68 therethrough in each cooling module location so that fluid communication is established between the plurality of cooling modules and the bottom tank. However, fluid communication is blocked between the hydraulic oil cooler module and the bottom tank by inserting a plug in the openings so that fluid within the bottom tank is not leaked to the atmosphere. This results in the ability to mount the hydraulic oil cooler module on the bottom tank along with the remaining plurality of cooling modules having a different fluid therein. Cooling efficiency is improved since incoming air reaches both the hydraulic oil cooler module and the remaining plurality of cooling modules simultaneously.