Fish tank is foggy and fish are dying?? HELP!!?

Okay i have a 55 gallon with 5 guppies, 4 neon tetras, 1 red-tailed shark, 1 dwarf frog, 1 pleco, 2 adult mollies, and 6 teenage mollies. My fish tank has been set up for about a year now, and has lately been getting very foggy, and today my healthy teenage molly died. what am i doing wrong? what should i do? i change 25% of the water weekly, i change the filter 1-2 months, i vacuum the gravel, my fish are fed once daily (pellets, flakes, algae disks, tetra color, brine shrimp, blood worms), i have a heater and the water is always around 78-82. what should i do?