[EDIT]

Hi Ive got a spare 40cm(l)x 30cm(w) by 25cm(h) spare glass tank, any ideas?

__________________
0.1 Bearded dragon (Sydney)
Reef tank
1.1 Jack dempsey cichlids
Hundreds of guppies!

im guess you mean inch? or cm.
If its cm theres more decetnt options. Given the huge space, you should look at colonies of something, cant think what i would want in that space.
Ants, Spiders, Scorps, pedes maybe
Bare in mind, given the general behaviour of these things, excuse the ants, you have to be very selective in species otherwise they will just eat each other.
What about some hermit crabs or something?
GL thinking! lol

__________________

Yeah, hermit crabs are a brilliant idea, theyre just so cute lol. Sorry, it is indeed in cm,
Any tarantula species suitable? Maybe arboreals (eg Avicularia) if i turned the tank on it's side.

__________________
0.1 Bearded dragon (Sydney)
Reef tank
1.1 Jack dempsey cichlids
Hundreds of guppies!

you got a decent range of spiders, only exclusions are gonna be the giants.
you could turn it on its side for an arboreal, and equally you cuold just stick a terrestrial.
I dont know your exp with ts

if your new id recommend a grammastola pulchra, they dont get too big, theyr very hardy, and docile.. and stunning.
Or a Avicularia sp 'guyana' for an arboreael, they quite small, really fuzzy, bloody cute and quick. more interesting to watch when they are out, but they tend to spend much time its own web construct.

If you know what your doing
id recommend a Phormictopus sp. they get a decent size, are stunning, and a bit more interesting to keep IMO than grammies.
Or for an arboreal, id so recommend Stromatpoelma calceata, tho theyr venom is reputed to be pretty toxic. the worst bite reports iv rad for tarantula have come from this species. They are damn quick, damn nervous, look scary, act scary. not for the feignt of heart.
They arent too big, but was always close to the centre of my collection when i had mine. When i can, il definately get more

__________________

Why not get a roach colony and feed your beardy?

__________________
Age and treachery will always win over youth and vigour!

what about mini ecosystem?
like a mini garden if you please.
maybe someone else can help more, but i was thinking a range of species, but all live together. maybe some colonies of ants, some ladybirds, worms, woodlouse, not exactly fantastic species, but then maybe some large fancy bug. just a thought.

__________________

giant millipedes will live happily with hissing cockroaches and/or fruit beetles-BEAUTIFUL CRITTERS!