OK so lets do a bit of math. The tank is 170g, a 7g water change is just over 4%. So tomorrow when you do your next 4% w/c, all things being equal, 4% of your w/c will be water from yesterday. 4% of 7g is a bit over 1/4 g. I don't see that as inefficient, more as not as efficient. But efficiency isn't everything. If you find daily 7g w/c to be a routine that suits you, or more manageable than weekly 10-20% changes, then from a workability point of view, this is the way to go. If both methods can be done, then do the testing that was mentioned. Of course, if you test before and after a 4% change the drop will be much smaller then after a 10-20 % this only stands to reason. To keep things on a level playing field, you would need to test after a week.
Myself I tried 2g daily w/c in a 40g with about 15g in the sump, the tank looked great, but I found it unsustainable. More to the point, I would forget to do them. I found it much easier to do the daily changes, but couldn't get into the routine. For me weekly w/c are the way to go.
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