kinda new tank question?

[st3]

i have a tank that has been sat up for about 4 months now with a whole lot of lr and just 2 mandarins in a 90 gallon tank. ive been topping the water off regularly and cleaning the filters. when am i supposed to do the first water change and how much water?? everything is alive and well (snails,crabs etc.)

 

[cadbury]

That actually all depends on you, how often do you want to do it? Some people do it once a week, 10% or you can do about 15% once a month, whatever works for you and your comfortable with!
Good Luck.