is it ok to top it off with tap water? Or Would I be better off getting a fresh gallon of cider and using that?
Well, I usually build topping up into my recipes as if you add water, you'll water down your wine. If you add fresh cider, fermentation will start back up and then you'll have to rack (and top up, and then rack....etc
).
Most people either make a bigger batch to start with, and then use various vessels for topping up the wine as needed. (I have a wine bottle, a growler, a gallon jug, etc). Or use the same wine from last year for topping up.
If you don't have something ready, the best thing is a similar commercial wine. For apple wine, I'd use some cheap white wine like pinot grigio. Or combine the batches, and use a smaller vessel for the topping up wine.
Edit- one other thing I just noticed! You may already be doing this, but cover your carboys with a towel or a blanket or something to protect them from light. It's hard to tell in the picture if you're typically doing that, so if you already are, that's great.