There are a number of fountains with a pool of water that can be placed inside or outside of player housing. These are all screaming to have fish. (I heard them just now.)
Choose only those fish that are like traditional tank fish (silverfish, goldfish are there any like poi?). Require them to be HQ. Let the recipe level determine how high a level the fish have to be equal to or less than to be placed. Let the recipe level or estate size determine how many fish can be placed.
And don't include fish within a fountain as part of the estate item count.
Or at least give Carpenters recipes for mounting lvl 50+ HQ fish on plaques for display in housing....