Question about critter spawning

I'm working on a barren planet (stable) to terraform it and introduce non-aggro life to it including critters like birds and fish (at least that was the intention?) but after spawning a few tests via fish and re logging I'm noticing that they don't like to stick around @.@ does anyone know a work-around for this? Any ideas or tips are greatly appreciated, thank you!

 

Critters and monsters seems to have the same spawn requirements: any place with large amounts of exposed background or for underground, any place that isn't heavily player built.
There's no way of stopping one without stopping the other.

 

I dunno, I've been building a pretty elaborate underground structure, and the poorly-lit sections tend to have monsters spawn in them. It's not the same in all the dark areas, though: the big, tall corridors have lots of monsters spawn in them, but the narrow ventilation shafts (still plenty wide enough to walk through) don't seem to spawn any. I think monster spawning may follow the Minecraft model of a certain level of darkness + a certain level of open space. Since lighting the structure better, I haven't been seeing monsters, though maybe that's a coincidence.

 

Well that actually confuses me, then, even more @.@ I'm using a dirt background to just spawn fish in. So I'm wondering if it's just a "native" block that they can spawn on or if creatures just aren't allowed to stick around on barren planets just because of the type of planet they are.
I don't think I've ever noticed creatures spawning inside structures before unless they popped into a structure I had just hollowed out of the ground and slapped a door on it for gardening and such.
I can't quite put my finger on the fish issue if it's a background thing because how else would the world hold the water?
Id like to also point out that I've been by a few generated houses on other worlds that have had hazmat suit people running around and they have built a glass structure in their houses to hold critters. So I'm wondering if it's possible or if that only counts because it was deliberately put in. And if so why tease us with the idea?