Not for goldfish I can't I'm afraid. If they have single tails you're going to need either a pond, or something about six feet, by two feet, by two feet, with big external filters. Common goldfish should grow to over a foot long, and are messy poo machines.
If they have double tails then a tank about 180 litres should be enough. Again you'll want a big external to run it. Either the Juwel Rio 180 or the Fluval Roma 200 would be suitable, but the small internals that they come with won't be.
Most tanks that come with hoods also come with filters, lights, etc. If you buy a Clearseal tank and a matching hood you'll still need to buy a light fitting for it, and you'd pay about the same for both bits as a good 60L tank kit like the Juwel Rekord 600, or the Korall (which is even cheaper, but has a hood with a feeding/maintenance hole that's very small, and a bespoke lighting unit that needs to be completely replaced whenever the bulb goes).
You live in Australia. Have you considered the native Blue eyes (Pseudomugil sp.)? P. gertrudae is probably a bit on the small side, but either of P. tenellus or P. signifer would be ideal, and they like water that's on the soft side compared to most rainbowfish and their relatives.
Don't worry about the carbon pads, they're only useful to remove meds or other impurities. If you leave them in too long they stop working and just provide surface area for the bacteria to grow on. Take the media out, chop it up and squdge it in. You should be fine.
So, I can make space for another tank. I have a choice. I have space for a 70cm tank.
MTS has bitten hard, I know.
I have typical London tap, rock hard (pH 7.8, GH not far off Lake Malawi/Tanganyika levels, nitrates pretty high too sadly). I can't fit an RO or nitrate filter to the water supply, so anything that goes in the tank must like hard alkaline water.
Unfotunately they don't seem to exist, but I can either move the 60cm (24") 54L Korall I already have and I'll have space for another 30" (80cm-ish) to go next to the current one. Or I can keep the Korall where it is, get a 45x30x30 (18"x12"x12"), move the least killies over to the new tank, and rescape the current tank.
Either option is fairly do-able. All I'll need is a cheap clearseal, underlay, and a clip-on light. I already have filters, and can move media around to seed them as necessary.
Current stock across the tanks is:
54L A colony of Least Killies. Either to be kept in the 60L or moved to a smaller tank (if I go down that route).
70L 30"x12"x12" River tank with 2x Darters, 2x Gobies, 8x WCMMs.
390L ND 4x2x2 Odessa Barbs, American Flag Fish, Zebra Danios, Weather Loaches.
I love loaches, but most are too active or need it too soft and acidic for my water. I also love temperate species, but I'd like to try something new.
I'd like to have variety across the tanks, and I want to do this on the cheap, (hence buying a cheap clearseal box) so I don't really want to have to buy another external filter/powerhead for another river tank.
My thoughts are currently one of the following:
A trio/quartet of Paradise Fish (will need a 30"). Alternatively put the AFFs in the small tank and Paradise Fish in the big tank. That'd give me an "Asian" themed tank (with lots of distinctly non-asian plants) and two US Native tanks.
Tanganyikan Cichlids. Either put some Multies or a similar shell-dweller in the 60L, or get the 30" and get those and a pair of the small Juli species and have a community. The downside? What if they breed?!
Danio erythromicron (or however you spell it). Another "species only" densely planted tank, full of tiny fish which I'm sure will end up looking like the Least Killie set up with different tiny fish/shrimp.
Axolotls! One will need the 24" tank, two definitely the 30". But the room may get a little warm for them, especially in summer. I've also got no experience of amphibians.
So over to you. What would you do given my water and limitations. Go wild!
Personally I would not worry. Fish lice do not look much like woodlice. What you're probably seeing are Water Lice, which are related to woodlice.
With regards to leeches it depends on the species. Lots of them, especially the smaller ones aren't parasitic. There are only a couple of species in the UK that will attack fish, and the best way of getting rid of them is to drain the pond and leave it dry for over a month. THe fish leeches that you're likely to see in the UK are fairly distinctive striped animals with large suckers. Most of the common leeches aren't anything to worry about.
The basic clearseals are nice "minimalist" tanks, (although not as pretty as the Dennerle one, or the expensive Optiwhite ones) and the black trim around the top is easily removable. You can then either build your own hood or place some coverslips on the top and clip a light to the side (or hang a lumiere from a ceiling/shelf above the tank.)
As for fish, as I'm sure you know it depends on what your pH and hardness is, and how big a tank you can fit in the space you have. If you can find them Variatus Platies and Least Killifish (Heterandria formosa) would both be good in a 60L (although not the same tank- Least Killies are tiny, and really only work in a species only set-up).
Add an external and a powerhead and you could keep Hillstream Loachs, Gobies, etc. They can be territorial, so you'll need to scape the tank to give lots of breaks in line of sight. White Clouds would go well with them, but they're pretty active so you need at least a 60cm long tank.
If you have space for something 2'6 - 3' long I'd also consider any of Paradise Fish, or American Flag Fish, or Odessa Barbs. Both Paradise Fish and AFFs are both aggressive, and AFFs will nip fins. On their own with suitable tankmates they're fabulous fish, and when coloured up Odessa Barbs will put many "tropical" fish to shame.