That Fishy Smell in Water

I think you use water for evaporative cooling like any other household air cooler. There can be only two sources of pungent smell, either air or water at the inlet point, but it will be a constant source.

As you said- "pungent smell in water and it becomes unbearable on Mondays", I'm inclined to think of another source. It can be smell emitting matter (like dead rodent) inside the duct which carry cool air to the shop. When system is not running, whole duct is filled with bad smell and you feel the punch most at the time of start every morning, later it gets diluted. "it becomes unbearable on Mondays" because smell emitting matter get more time due to Sunday off and its concentration (of pungent smell) increases. So inspect the duct.

Mold and mildew and dirt/dust building up in system. Is it evaporation belt system, air mist, cooling tower?

Water needs to be treated, system cleaned, system sealed and proper air filters used.

The use of perfumes can cause other problems for people sensitive to such.

If the duct is fiber board or fiber insulated inside is may beyond fixing. High humidity air may have saturated it and mold mildew had set in. If metal or fiberglass ducting it may be cleaned.

There may be low spots that had collected water and become breading grounds for mold/mildew.

Like said before, dead animal or there wast is breading grounds also.

You should find the problems and get it fixed before people get sick and/or die from contracting an airborne disease.

Good luck with it.