I think I'll get a 150, because if it goes off I'll have less time to realize.A 29 US gal tank is about 110 litres so using the minimum 1 litre to 1 watt for heater I would suggest a minimum 100watt heater but I would go for a 150 or 200 watt heater.
A higher wattage means the heater does not have to work so hard to heat up the tank water so therefore cheaper and better for the heater.
Heater simply only goes on when water temperature goes below a certain temperature and stays on until water reaches desired temperature so it won’t overheat.
I would suggest a good quality heater don’t get cheap heaters, as I think heaters and filters are the only two tank equipment that must not be skimped on tbh.
Fluval E series heaters are good and I’ve had them before, would recommend those.
What frustrates me the most about some of these cheaper heaters is that they have a dial at the top where you can turn it to the temperature you want - that is if you can read it. The top is black and the raised numbers are black. I bought one that looked like it was black with white numbers and bought it - turns out it's gray with white lettering - very hard to distinguish. I don't get it, do they think we all have perfect vision? In order to read the white on gray it took me wearing my glasses AND using a magnifying glass so I could set it at the temperature that I wanted. Two days later (after a week of 80 degree weather it decides to go sub-freezing with snow and sleet. (typical Kansas weather). So I needed to readjust the temperature on the heater. I finally had to unplug it, let it cool off and then pull the whole thing out of the water to make the adjustment with my glasses and magnifying glass. This is RIDICULOUS - do they not do any user testing on their products before they put them up for sale? When I was a project manager for web applications we conducted usability testing by developing our application then having a random group of users try it out - we could see where they were confused, and they could tell us why and we got immediate feedback. Most of the fixes took less than a day and we ended up not having one person mess up their selections when we went live (over 30,000 employees had to complete this).
I've been using some expensive heaters that include with the heater, a red display of the temperature in the tank. Your controls are outside of the tank and you can just click to adjust the temperature (actually it just goes up to about 90 degrees then drops down to 50 degrees so it's a lot of clicking to get it to 78 or 82) but I can READ the display and that is what is important to me. One did go bad within 30 days so at least Amazon handled the return and I had them credit my account while I purchased an overpriced crappy design heater at a local fish store. Can't go without a heater in Kansas weather in October. (really, the price on this heater was 2x as much as on Amazon - so for that much savings you could afford to buy a spare from Amazon just to keep on hand.) - On a side note, if they used a color like blue it would be much better. There are a number of red/green color deficient people that may have problems reading the red lights. There are also blue/yellow color deficient people but this is far more rare. Using a color like blue, however unfortunately correlates to cold water when you actually may be making it warmer - so perhaps a bright green with a high contrast to the black would be best for the red/green and other color deficient folks. My husband had red/green color blindness but if the contrast was strong enough or the light bright enough he could easily read a red/green display.
So - my rant is about over, but let's face it one of THE MOST IMPORTANT and DANGEROUS pieces of equipment in your tank is your heater. The manufacturers need to take it more seriously and we, as consumers need to insist on it. If you can't read black on black - send it back and tell them that. All they need to do is change it to white numbers on black - maybe it's a big deal. But for now I'm sticking with the heaters that display the temperature in bright numbers and are easy to set. RANT OVER.