ah, the magical word of 'depends'!
if you dont do anything to your tank except add fw topoff, you are headed for a crash...maybe.
if you keep nothing but a refugium and add fw topoff only...you are headed to a crash...maybe.
if you have a skimmer, refugium and do nothing by fw topoff...you are headed to a crash...maybe.
there will always be maybe's in this hobby.
probably a good way to avoid problems is to have a refugium, a skimmer and do water changes. in fact, you dont even need a skimmer and refugium IF you find a way of exporting waste and nutrients...with water changes being the usual method.
skimmers are great for getting rid of waste and if you skim wet, you can get rid of them before they break down into problematic dissolved oprganics.
a refugium is great at growing macros that will soak up the guck that the skimmer missed...
but...i would still recommend water changes because there are compounds that a skimmer and/or a refugium just can not export. plus, water changes bring back into ionic balance what your corals etc have used while leaving other elements behind...
what does all my jabbering mean?
again, it 'depends' on the system you want and have.
my 180g sps tank is skimmed very heavily and get a 70g water change per week. my 110g softie tank has a small refugium gets moderate skimming and much less water changes and my 125g fowlr gets light skimming a very effective refugium and no water changes. all do great with all receiving different treatment. note that they are all different ecosystems, each having different inhabitants and different goals.
i analogize it as such:
- you go the a vegas 5 star french restaurant and eat haute cuisine and while not hungry...youre not busting at the seams...
- you go to vegas, hit the $30 buffet and fill up on crab legs and good beef...youre fully, really full...happy full...
- you go to vegas, cheap out on the 5.99 buffet of mystery meat, greasy deep fried shrimp and feel not only full...but really ill...
the similarities are vegas and food...the differences are quality of food and setting...the quality of food determines how i feel (good to ill) and the setting makes me feel like a king or a guy who just lost his bankroll.
where am i going with this vegas thing? i have no idea...