Hi Guys

Im going to be starting a new job on Wednesday and its a night shift, 12 hour shifts at that. Anyways I know im going to feel very sluggish for the first couple weeks and just have a question about my water change.

I try to change 30% each week in my 40 gal goldfish tank and was wondering If I was to do a 50% change tomorrow will that buy me a couple weeks until another water change or is that too much to change at once?

any tips would be great