Your idea is amazing, and I'm thinking that many of us have dreamed of something similar at some point! Hopefully some of our members will be able to get you get the input you need to get started. . .
Questions for YOU first. . .
Is there ANY
chance you can try this amazing setup with slightly larger tanks?!! Just upping them from 5g to 10g or 15g will give you so
many more options. . . and make your life much easier as far as maintaining consistent parameters (including temperature!). Especially with saltwater. I'm not terribly familiar with the salty side, but from what I've seen poking around, you may find it fairly difficult to maintain a nano tank of this size as a beginner. . . Personally, I'd rather go down to ONE tank of each type and have it be larger/healthier - Though that kind of kills the idea of tropical/temperate tanks for each! :/
Do you have the tanks already? If so, wondering the dimensions - the difference between a standard 5g and a 5g hex (or whatever!) can make a big difference in the suggestions you will receive!
What is your water hardness? Knowing your kh and gh will help us out a bit as far as offering possible suggestions. . .
Along the same lines - how willing/experienced are you with shifting and maintaining water parameters in tanks of this size? Most of the smaller/nano fish tend to be very sensitive, and as the tiny tanks are so subject to flux. . . it can be very dangerous for them, unless you have a fair amount of experience with these things. . .
I'm wondering how set you are on FISH
- or would you be willing to go with other fun things, like shrimp, frogs, crabs, even maybe a coral (if that's possible, idk!!!) would make a pretty cool display, and could be far easier for you to maintain.
Does it HAVE
to be 3-5 DIFFERENT species? Speaking from a freshwater perspective, there are probably a few nano fish that can be kept in a well maintained 5g (10 would still be much better!), but some of those need to be kept on their own as specemin fish, and most others require a shoal - in both cases you'd be stuck with a species-only tank. I'm not really coming up with many COMMUNITY
nano fish that you'd be able to stock in a 5g tank, unfortunately. :/
As far as setups/aquascapes - that's always fun! But this will likely be directly related to the animal(s) you end up with - so better to figure that part out first, and in the course of researching, you'll begin to form a clearer image of the needs of the creatures who will be living there, and can set the tank up as a biotope to best suit both them and you!
Welcome to TFK, by the way! Glad to have you - and I'm not trying to rain on your parade!!! Hopefully we can all work together to find something that will work for both you and the animals, so everyone can be happy! In the meantime, I suggest that you pop over to our Beginner Saltwater
area, and set up a thread there to get more input on what you'll need to set up and maintain nano saltwater tanks of this size!
Really looking forward to seeing this dream develop, and wishing you the best of luck on this fun project! ^__^
ETA: I'm going to move this thread into our advanced discussions area! This setup is going to require quite a bit of discussion, and a skill-set that's more advanced than what your average beginner has to properly maintain! Typically, our suggestions for beginners wanting to set up tanks of this size would be good ol' Betta splendens, and I'd hate for a true newbie to see a recommendation for a more sensitive fish and not have the experience to keep it well!