The branded ones are also made in asia 🙂 but i had one and indeed the thermostate was rather inacurate. Dont know the acuraty from branded heaters my last one was over 20 years ago..
Dont know the size of your tank. But if you lower the wattage it might be sufficient but then youll need to go realy low.. Like i used an 80 watt heater on 50 liters and the top temp i could reach with that was 26 degrees even if the thermostate was on 28 whole day long. But keeping a tank at 18 degrees is pretty low and not far from the invironment temperatur. And like now in the summer you more like to need a cooler than a heater to get to that. The water temp in the tank will somewhat level with the invironment temp it is in. For example my small tank is 27 degrees now, it was a warm day, the room it is in already went down to 26.1 degree. But tank is still at 27, the heater hasn't been on for weeks.
Another option would be to add a second electronic thermostat one with a relay powerfull enough to switch 220 volts and the amps of your current heater. If you put the heaters own thermostat on 20 degrees. and the second thermostat on 18 degrees which is switching the main power of your heater. So if the water reaches 18 degrees the main power of your heater is cut off even if that one is put on 28 degrees doesn't matter.
I use a small cheap unbranded electronic thermostat like that to switch my heater.. And it's pretty darn acurate on the 1/10 of a degree.. It runs on 12 volt but it's relay can switch 220 volts up to 20 amps.