have have about 20 fish 2 angels 4 guppies two big tetras, two bristle noses, kuli loach, coradoros, 3 mollys, swordtail, lemon tetra, 3 black widows, a phantom teta and a few others. i am going to buy a new tank in the summer and sell this one, that i have got now.
I think it very much depends on your stocking levels and tank setup. But I clean mine typically every 2 - 3 months or so - or sooner if I see a reduction in flow.
I have a few pooy bums in my tank these days so I tend to change the white sponge every 2 months, rinse the blue pad out fairly well & a very light shake of the three baskets in the filter water. Then totally clean out all the shet from the bottom of the filter with a damp (Dipped in tank water cloth). I've got an Eheim 2128 Thermo external.
Unless my eyes are failing me, I cannot see how my gallons / litres your tank holds
There are several models in the professional range - which model do you have ?
Also, what media are you using in it ? the recommended efimech and Ehfisubstrat + sponge pads ?