75 gallon: Lots lately. 25~75% water changes. Normally, I just set up the flow so that the filter pumps out at a certain rate, and the I set up temp matched water from the tap thru my Aqueon siphon and set that up to water fall into the opposite end of the tank, then let that run for 20 mins or so. Worked pretty good. With recent events, I have been using a bit more involved process, as I am using aquarium salts and will be soon phasing that out.
38 gallon: Have not done a "water change" in 2 years now. I add water when it gets about 2 inches or so below the trim, before the line on the heater for minimum level is reached. This tank is actually, by some accounts, overstocked. It has perfect parameters and is seemingly my healthiest aquarium. I have not gravel vac'd this tank in.over 3 years.
30 gallon: I sometimes do a 20% water change on this tank, but not very often. I usually just add as it evaporates here as well. This tank is understocked, and I feed sparingly in here, so, the params stay perfect here as well. It is one of them "that shouldn't work" tanks as I have an undergravel filter on here, heavily planted and have not done a gravel vac in over a year. This tank is a giant biodome. Is another type of experimental ecosystem I built.
10 gallon: I add to this tank whenever it gets low. It is heavily planted, and has 2 fish in it. I only do a "change" when I suspect issues in there. So far, after 6 months, there have not been any. Water is pristine and excellent params.
Some have their ideas on how to handle an aquarium. Would I suggest what I do to new people? No. You have to monitor things and do things that are...unorthodox. Recently, my lack of monitoring, caused me to lose 2 of my big fish. A hard lesson, but it reminded me to keep tabs on what is going on. I have begun to build this tank into an eco system as well now. But the rest of my tanks are self sufficient. They require minimal work to maintain. I have populated my 38 gallon in such a way as it takes care of itself. This takes practice, and lots of knowledge of how the actual environment you want build will work. Right down to the cycle.