hhmm..you dont want kitty falling in either!
my parents had a huge marine tank & the cat (a very big cat i may add) managed to get inside, cracking the glass with the added pressure & crash everything came down around her..(thank god as otherwise she may have drowned) but this meant thousands of pounds worth of damage..fish..everything gone..& that also meant the house was flooded...so onto the insurance company we went..which meant new carpets throughout the house...i could go on..but i'm having an image of neptune being only little in a little tank..lol
but i thought you'd like my story...
more to the point, i should think kitty (if kitty even notices neptune) wouldn't be able to get up that high...unless kitty made it kitties mission! i did know of a friend who had the same idea as you but her (grown cat) did get to poor bubbles the goldfish
& he was found weeks later behind the dresser...someone had had a great time playing with him! although bubbles may not have agreed with that statement..
i'd say dont worry too much..but do keep an eye on your kitty..especially if kitty has an eye on neptune!