Assasin Snail Clea helena, a freshwater welk. Will eat juvenile Physa (small pond snails), Malaysian Trumpet Snails, and also acts as a scavenger. Will breed, but you need males and females as the sexes are separate (unlike the MTS's where they're all females and reproduce through parthenogenesis, or the pond snails which are hermaphrodites). They reproduce fairly slowly, so they won't build up into pest proportions.
From what I read they should be fine with Nerites too- they're too large for them to attack, as are adults of the various Tylomelania species. Babies on the other hand are another matter.
The only downside I can see is that they'll probably attack fish eggs, and possibly even the less mobile newly hatched fry. So in my Spotted Rainbowfish tank I'm still equivocating on whether I should add them- I've got a fair few juvenile pond snails, and lots of shrimp. And if the fish are going to breed I'm not sure if adding yet another potential egg-predator to control one I already have is going to be a good idea.
There's really no fish you can keep in a 30L. Bettas need 45L (and a heater) the related Paradise Fish (Macropodus opercularis) will live in an unheated tank too but they need similar -if not bigger- water volumes.
White Cloud Mountain Minnows are shoaling, so you'll need to keep 6 of them, and that means a 50L tank too I'm afraid.
Either would be excellent choices for the 60L though. I'm not sure they'd share too well.
Shrimp however will do absolutely fine in the small tank. Four or five Red Cherries and within a couple of months you'll have a swarm.
Re: Question on stocking tank from 'a guy called adam'
Depending on what you end up keeping you might be able to press it into service as a fry tank, or a QT/treatment tank.
Bear in mind that small volumes are much harder to keep stable. You may find that temperatures fluctuate more, and being a smaller volume of water if you do get an ammonia/nitrite/nitrate spike there's a lot less water for it to be diluted in, giving you less time to deal with the problem before things get serious.
re: Ideas please - we have to have a complete rethink!
Check out the sticky at the top- there's a link to the WetWebMedia article on hard water fishes.
You might want to look at Rainbowfish. There are a wide variety, in many sizes, and they're mostly tolerant of hard water.
In a 90-100 litre tank you could keep a reasonable sized shoal of Forktailed Blue-eyes (Pseudomugil furcatus), Spotted Blue-eyes (Pseudomugil gertrudae), or Pacific Blue-eyes (Pseudomugil signifer).
None get very large, so you'll need to think carefully before adding tankmates- they're probably better in a species tank, perhaps with something bottom-dwelling and peaceful for company.
There are lots of other livebearers out there to try, not just the guppies, as well as several species of Danios- but they'll need a relatively long tank (3 feet or so) to swim in, they're very active.
There's always the old staple of White Cloud Mountain Minnows (Tanichthys albonubes), which are lovely and won't need a heater.
Unfortunately 5 gallons is just too small for almost every fish- either because the fish themselves get too big, or a tank that size won't support enough to make them comfortable.
Cave Fish get too big, up to 9cm. At the 1 inch per gallon rule, that means you need about 4 gallons per fish. So in theory you could keep one and a half in your tank. However they're schooling fish, so you really need six of them, which means a 90 litre (20 gallon) tank. They're also surprisingly active even though they're blind, so you need to bear that in mind too. (I've learnt that lesson the hard way). They're fascinating animals though. One day I'll set up a dedicated cave tank for a shoal...
I think the tank is probably too small for White Clouds too, they're a shoaling fish too, so you really need 6, and a 10 gallon tank to keep them happy.
You could keep a single Betta, though they (like the Cave Fish) will need a heater.
Personally I'd plant it up and breed shrimp. Red Cherries are cheap, fun to watch and I can't seem to stop mine breeding. If you have relatively soft/acidic water you might want to try the more sensitive Bee or Red Crystal shrimps after you've cut your teeth on the RCS's.
There are three species of shrimp commonly sold. Like fish they won't tolerate any amount of ammonia or nitrite, and copper (in medication or fertiliser) will kill them stone dead.
Caridina japonica (Caridina multidentata) Amano Shrimp Gets to about 2" for a female, 1.5" for a male Will not breed in aquariums, the larval stage requires salt water. Temp 68-82?F/20-28?C pH 6.5-7.8 Transparent Grey-Green, with brownish spots forming broken lines and a white stripe down its back.
Neocaridina heteropoda Red Cherry Shrimp, Yellow Shrimp Gets to 0.8-1.5" Temp: 62-82?F/17-28?C pH 6-8 Breeds very readily (when I moved my shrimp into a larger tank I found I had well over 100 juveniles of them from two pregnant females. Larvae are smaller versions of the parents. Mine seem to happily tolerate London water with a hardness of about 15 degrees.
Caridina cantonensis Bee Shrimp, Crystal Red Shrimp, Tiger Shrimp, Black Tiger Shrimp... Lots of varieties some very expensive indeed. Gets to 1-1.2" Temp: 64-78?F/18-26?C pH 6.5-7.5 Lots of different varieties, and again breeds readily in freshwater like N. heteropoda. Being the same species these varieties will happily interbreed, so should be kept separately. It won't breed with N. heteropoda though. Prefers much softer water gH 4-6, kH 1-2 than the others will tolerate (I've not tried any yet for that reason). Some varieties are very inbred and even more sensitive to variation. While the wild-type "Tiger Shrimp" might cope a little outside this range, the high grade Red Crystals or the Black Tiger certainly wouldn't.
All shrimp will happily eat hair algae and the various micro-organisms living in it, with regular supplements of algae wafers, flake, bloodworm, shrimp pellets or blanched vegetables- they'll happily scavenge on what your fish don't eat, but if you keep them alone, they'll starve if you don't feed them.
They should be fine with snails (I've seen mine happily picking algae off the backs of them), but I'm afraid I don't know anything about Bristlenose Plecs. As a rule if it can fit the shrimp in its mouth it'll probably try eating it. Even if it can't it might well pester them to death nipping at it.