As of today I have 8 tanks running, 7 salt and 1 fresh.
I tend to spread out my mainetenance when I can. I will water change the 125g salt one day and then do the 29g cube and 60g cube on another, the 90g I do whenever I can. the smaller tanks I have I will do whenever I think of it as they are small and a bucket or two of water is generally all thats needed.
It is a hassle to behonest and while I love all my tanks I am planning on consolidating the 125g, the 29g biocube and the 60g rimless into the new 180g tank I just setup.
I am torn as to what is easier though, the 8g pico I have is simple, a couple scoops of water and the water change is done. The 125g is a production to do a water change on but the tank is far more stable as far as parameters than the 8g pico.
Good luck with your new tank(s)