My treadmill belt appears to be slack and when I try to tighten it it still does not run properly it tends to slow down. Should I change the belt

The belt tension is fractionally too slack.

first of all check the drive belt is loose or not. Drive beit connect with the motor and roller of the treadmill. its not possible to see the drive belt out side. you can only see it if you remove the motor cover. if its loose the try to tighten it. if its not the problem then the treadmill motor have some problem. Either the motor is faulty or the carbon brush is faulty. There is 2 carbon brush inside the motor which is replaceable, its used to take the voltage to the rotary part (armature) of the motor. If the contact with the rotary part and carbon brush is poor the this type of problem can occur. tyr to change the brush. Here i have added a video which demonstrate how to change the carbon brush of the treadmill motor. It not the exact model you have. but almost all treadmills have the same type of motor. even after replacing the carbon brush the treadmill shows the same problem then you have to charge the motor with a new one .
http://electro-medical.blogspot.com/

Fitness repairs

if your walking belt is slipping it has loosened up. there are two adjustment bolts on the end of the treadmill. one on either side. these are allen heads. tighten clockwise to remove the slack. first start with treadmill running at 2 mph. tighten both one full turn. let belt adjust. check the centering of the belt. whichever side it is closer to tighten that side about a half turn. let belt adjust after each adjustment for a min.once belt is centered and if it still slips tighten both bolt evenly

Loose is not the answer. Proper lubrication and correct tension adjustment is the answer. I don't know how many HP the motor is for, but for 265 pds it should be at least 3HP.
Put the speed at about 3MpH and while walking, make a small jump on the belt to ensure that it is properly tensioned. Needless to say that the belt must be well lubricated. If you see that the belt tends to stop when you make the jump, turn both allen screws exactly the same amount clockwise. Half a turn might be enough. When you're satisfied and the belt doesn't seem to stop, give both allen screws another half turn clockwise.
All that as long as the treadmill is capable of handling your weight.

From the manual: Troubleshooting
Symptom: The walking belt slows when walked on
a. If an extension cord is needed, use only a 14-gauge general purpose cord of five feet or less in length.
b. If the walking belt is over-tightened, the treadmill performance may decrease and the walking belt may be permanently damaged. Remove the safety key and UNPLUG the power cord. Using the 3/16" allen wrench, turn both rear roller adjustment bolts counterclockwise. 1/4 of a turn. When the walking belt is properly tightened, you should be able to lift each side of the walking belt 3-4 inches off the walking platform. The center of the walking belt should just touch the walking platform. Be careful to keep the walking belt centered. Plug in the power cord, insert safety key and run the treadmill for a few minutes. Repeat until the walking belt is properly tightened.
c. If problem persists, contact customer service.

f the walking belt slips when walked on, first remove the key and UNPLUG THE POWER CORD.Using the hex key, turn both idler roller bolts clockwise,1/4 of a turn. When the walking belt is correctly tightened, you should be able to lift each edge of the walking belt 2 to 3 in. (5 to 7 cm) off the walking platform. Be careful to keep the walking belt centered. Then, plug in the power cord, insert the key, and carefully walk on the treadmill for a few minutes. Repeat until the walking belt is properly tightened.

If the walking belt is overtightened, treadmill performance may decrease and the walking belt may become damaged. Remove the key and UNPLUG THE POWER CORD. Using the hex key, turn both idler roller bolts counterclockwise, 1/4 of a turn. When the walking belt is properly tightened, you should be able to lift each edge of the walking belt 3 to 4 in. (8 to 10 cm) off the walking platform. Be careful to keep the walking belt centered. Then, plug in the power cord, insert the key, and run the treadmill for a few minutes. Repeat until the walking belt is properly tightened. . If the walking belt still slows then lubricate running deck the lube may be purchased from iconfitness.com

The walking belt slows when walked on the walking belt maybe overtightened, treadmill performance may decrease and the walking belt may become damaged or the opposite the drive and /or running belt may need tightening. Last thing would be that the belt needs lubrication to be applied between belt and deck.

If you have not ever lubricated the running deck then I suggest doing so . What happens is that the lack of lubrication will tend to make the belt drag thus slowing down the belt making the motor control board to try to compensate the drag.

Use only a single-outlet surge suppressor that is
UL 1449 listed as a transient voltage surge suppressor
(TVSS). The surge suppressor must have a
UL suppressed voltage rating of 400 volts or less
and a minimum surge dissipation of 450 joules.
The surge suppressor must be electrically rated
for 120 volts AC and 15 amps. There must be a
monitoring light on the surge suppressor to indicate
whether it is functioning properly. Failure to
use a properly functioning surge suppressor could
result in damage to the control system of the
treadmill. If the control system is damaged, the
walking belt may change speed or stop unexpectedly,
which may result in a fall and serious injury.