Great update, should be a quiet room. The HRV is that way to go for your fish room, it's pricey but well worth it. If you connect it to your furnace(forced air) the furnace fan has/should be run continuously for proper air circulation,(this is what I was told) different models may have different requirements though; this may be an added electrical cost.
Another option would would be to install an exhaust fan in the fish room, the
Elicent inline fans work great and move a lot of air for the money.