You have it backwards. Trichlor is acidic and will reduce your pH. Muriatic acid will reduce the pH (and alkalinity), and you need to get your pH down within 7.2-7.8. Here are some good instructions: http://www.poolforum.com/pf2/showthr...Your-Pool-s-pH

Also, make sure your filter is clean and run the filter 24x7 until you have things cleaned up. Chlorine kills the algae, and the filter removes the dead algae from the water. At 40cya you need to be between 3 and 6ppm fc for normal operations. To clean up the algae, you need to bring the pool to "shock" level and keep it there until you can go from sundown to sunup without losing more than 1ppm fc between the tests. Shock level for your pool would be at least 15ppm. on 15.5k gallons, that would be just under 4 gallons of bleach. Remember, that for this to work you have to maintain that concentration by testing and adding more bleach often (a few times a day).

To determine your operating and shock levels, read this: http://www.poolsolutions.com/gd/best...ine-chart.html