when people ask advice on tank sizes i always say the biggest you can aford and have room for. now part of me is obviously fish obsessed and likes nice big tanks, however that's not the only reason. a larger body of water is much much more stable, and therefore much easier to look after. small tanks the levels will fluctuate and it can be a nightmare.
it does seem poor goldie is stunted, however you're doing what you can now to make the rest of his life happy and peaceful and that's what's important now.
depending on the size of the tank you get you may need to get a new filter but by using it in tandem with the old filter you won't need to worry about your water levels, should go smoothly. Your filter is rated for tanks up to 45 litres, now the ratings are usually a bit optomistic so you can say it's really suitable for a 30-35 litre tank. So if you get something bigger than that you'll need to get another filter for it, just find out the size of the tank in gallons or litres and any filter you look at should have a rating on it to say what size tank it is suitable for. Brands I recommend are Eheim or Rena, fluvals are OK but they don't seem to last as long as the eheims.
here's my step by step guide to transferring tanks around
1 - set up new tank, make sure it is level and sat squarley on the stand (v important, if it's wrong the tank may crack at a later date and make a right old mess!)
2 - set up heater, new filter lights and any other equipment on the tank, just put them in place, do not switch them on
3 - fill new tank three quarters full with new water from the tap, add dechlorinator
4 - switch on the equipment, leave it running for 15 mins or so before doing anythng else, just to make sure there's no obvious problems, if you're getting a heater wait until the tank has reached the right temp.
5 - add your gravel (make sure it's washed first) and any plants or decorations you've got for the tank
6 - catch goldie and transfer him to the new tank. best way to catch fish is to drain the tank to half full, remove all decorations, get two nets one big and one small, hold the big net at one end of the tank, using the small net chase the goldie into the big net, when he's in swiftly pull it out of the water, gently hold the goldie through the net so he's not flapping around as he could hurt himself. as quickly as possible transfer him to the new tank and just plop him in
7 - turn off equipment on the old tank
8 - take the filter off the old tank and move it onto the new one, set it up and switch it on
9 - turn off the lights on the new tank, don't feed the fish for 24 hrs. keep an eye on him but you can expect him to be a bit disorientated and not exactly on top form at first, it's fairly stressful for them to move tanks, he should perk up in a day or two though.
10 - then strip down the old small tank, drain the water and sort everything out to be sold/thrown away as appropriate
you'll need to leave the old and new filters running in tandem for around a month, this means the bacteria from the old filter colonises the new filter. you can then remove the small filter (although you don't have to, it won't do any harm to have 2 filters running, plenty of people do it so that if one filter fails you have another one there doing something still)
hope that helps
