When I went to buy the pleco, I tried to buy one that was roughly the same in length or longer than Koopa. At that time the pleco was a little longer than Koopa was but now they are both about the same (2 inches).
A heater is extremely important to have even year round because water temp should remain constant 24/7. If it fluctuates, your turtle runs a higher risk of getting sick. Do not rely on your lights to stabilize the water temp. 78 water temp for hatchlings, 75 for older turtles with the basking area around 90 while the lights are on.
Here's a couple of heaters that are recommended: Visitherm Stealth heater and Ebo Jager heater and for wattage figure 5 watts per gallon of water. Knowing the temps of both the water and basking areas at all times will help prevent illness from occuring, so when you buy a new themometer you may want to invest in a couple extra to always have one on hand when needed.
Btw, how much water do you have in your 29 gal now?