Working with two aquariums lately, a 55 gallon and 20 gallon. Each one a home to a approximately 7" Red-eared slider.
Filter of coice on each is a FilStar XP-M aka XP2. On the 55 gallon I vacuum twice at least twice a week which ensures some water gets changed. Any surface oddities can be resolved with a bubbler or by reducing the water level by 1 inch to allow the agitator to agitate the surface properly.
The question really concerns the 20 gallon. Yes I know, poor girl should be in a larger tank. It's in the pipeline, priorities are currently focused on strengthening her up and preventing illness. The water in this tank, is crystal clear since installing the filter nor does it smell.
If the water is clear and doesn't smell can I simply replenish the evaporated water or should I actually remove some even if there is nothing to vacuum?