Tank size and water temp fluctuations

[BetterWithBettas]

Hi all,
Just wanted to know your experiences with tank size and water temp fluctuations.

My 20Litre/5G tanks are fluctuating too much for my liking and probably my fishes liking more importantly, so I'm wanting to upgrade the tanks soon to help with more stable water temps. Both are running off 50W heaters and I've tested a few different ones to make sure it was not the heater bugging out.

What's the smallest size that is bigger than 20L which will keep the water temp more stable ? The space that the 20L tanks are in is fairly limited so the tank size if I'm wanting to keep them where they are is also limited to some degree. A 30L /7.9G would be a good fit. Would a 30L be enough water volume to have more stable water temps? Or will I run into the same problem?

Thank you for taking the time to read and for any advice/experiences offered

 

[kwi]

Is the heater horizontal and at the lowest point in the tank and in the current flow so the water is as evenly heated as possible?

 

[BetterWithBettas]

Is the heater horizontal and at the lowest point in the tank and in the current flow so the water is as evenly heated as possible?

Yes the heaters are horizontal and the hob filter flow is and intake is running over and near the heater