Honestly, you could get by by doing 20% changes every two weeks. That is the exact amount I do on all my tanks, planted, African Cichlid, everyone. I have found it's alot easier on the fish and plants at that. Since less water from the established tank is being removed they are not nearly as stressed out as larger changes over longer periods of time. When you siphon out the water in the tank you are removing alot of the minerals and properties of the water (temp, phosphates, beneficial bacteria, ect) and when you add the new water, yes it's dechlorinated/right ph/right temp, but it does not usually contain all the other stuff that the tank needs to sucessfully thrive. Another thing that happens when you do bigger changes is the distrubance within the tank. Depending on how fast you add the water it might throw the fish around by the force of the water being added to the tank which might cause just enough stress for other minor factors within the tank to thus activate.
So in the end, I would suggest doing a smaller change. Like I said, unless you are having a breakout of algae, bacteria, or diseases, 20% water change every 1-2 weeks is plenty enough to keep everything fresh. Honestly if the tank is not having problems and everything is thriving well, you could just
"top off the tank" which is just adding enough treated water to the tank every few days to a week to compensate for the amount of water that has evaporated. If you do this water changes still need to happen but you could do it even less often like once every 3 weeks. The reason you would still need to do the changes is because when the water evaporates, essentially thats all that is dissappearing. The minerals and such that are in said water that evaporates is being left behind when the water evaporates. For example (by no means this to be accurate, just numbers to use as an example), if your tank has 50ppm of calcium, and say one gallon evaporates, there is still 50 ppm of calcium in the tank. Now when you add say one gallon of water from your tap that has 5ppm of calcium in the bucket, you now have 55ppm of calcium in the tank. The problem that results from this is a buildup of said minerals to a deadly point. But as long as you test your water frequently you will be able to catch the levels before they get too high.
So here are the basics on what might help your tank go smoother in times of water changes
-20% water changes every 2 weeks
- Top off with treated water, 20-35% water change every 3 weeks
-Be easy when adding the water to the tank, less disturbance, less stress on inhabitants
Hope this helps!