Thanks so much for all the input! I think I will see if I can talk my husband into the idea of a bigger tank, especially since there's that $1/G sale. Thanks also for the reassurance about cycling - maybe it is something I can manage after all.
Thanks also for the suggestions about ways to find a more experienced fish sitter in the future. I am pretty annoyed with our fish sitter bc not only did she not siphon out any of the waste like I asked her to, but she also admitted that she fed the fish more than I told her to because she felt bad for him knowing that he was on "half rations" - after I specifically explained that the point of feeding him less was so that he would produce less waste and his water would stay cleaner! So, basically, she killed the fish and doesn't even realize it.
This may be a dumb question, but could a 5 or 10 G tank be supported by a regular piece of furniture, or does it require an aquarium stand? We have an ideal spot for a tank on top of my husband's dresser (gets natural sunlight for part of the day, is away from the kitchen & cooking fumes - which I think got one of our past fishes sick - and quiet for much of the day but with plenty of human interaction in the morning & at night). I would like to keep the new tank there as well but not sure if we're getting into territory where it would be too big?