The temperature, pressure and radiation are all not optimum on Mars for life. But if the pressure were several fold higher (20-30 mbar instead 6 mbar), the temperature 5C higher, and with less radiation could we manage an open air lake seeded with all the life needed and fish? This could provide food for a colony but I want to make sure it is doable.
You can not have an open air lake on Mars
Lower gravity causes gases on Mars to escape off into space. It's solid core also means that Mars has no magnetosphere so solar wind speeds up this process. Since you are assuming slightly higher air pressure and slightly less radiation, I will suppose you gave mars a molten core, but your low gravity will still be a very big problem. On Earth, water vaporizes, gets caught in the atmosphere by gravity, and eventually becomes dense enough to form rain and come back down.
Mars will still have evaporation, but the air will never get thick and cold enough at the same time to precipitate; so, an open air lake will quickly dry up.
As for Fish as a Food Source on Mars
If you want an "open air" lake, it will need to be part of some contained habitat. Keep in mind however that open lakes are REAL inefficient sources of meat. In a natural lake, most of the biomass is not things that humans eat; so, you generally only get about 25-100 lb of meat per acre per year. For comparison, 1 acre can sustain a production of about 1700 lb of chicken per year or about 3000lb of wheat or soy or 50,000 lb of potatoes making open pond fish farming a bad source of food anyway.
If you insist on fish, you will want to really min-max how your raise them. If you have a catfish farm, you could use reflectors to heat vertical glass tanks, and add sewage to encourage rapid algae growth providing a good food source for a dense fish population. Mechanically aerating the water is also a must. This way you can get a good sized meal fish that eats an easily grown high energy primary producer. Done right, you could probably achieve close to chicken levels of efficiency with fish, but not in an open lake.