For five months me and my friend are doing a vegetarian/fruit/fish diet only. We eat fish every day (salmon, shrimps, tuna). We now what we eat low mercury fish but can these fish cause cholesterol lvl to be high? Any side effects?

Omega 3. Omega 3 found in certain types of fish improve cholesterol and also help reduce inflammation that is the initial cause of atherosclerosis. No worries with increasing cholesterol eating fish like you're doing.
Eating Fish Daily. I commend you for your desire for Good Health. American Heart Association recommends two serving of fish in a week. By eating fish every day, there can be more LDL Cholesterol; it will decrease Triglycerides. Tuna fish can have mercury. Salmon, Sardines, Mackerel, and Trout are full of omega-3 fatty acids -- good fats. Add walnuts, flaxseed, canola oil, and omega-3 enriched eggs. Exercise daily.