First of all, read the stuff in this link about cycling the tank:
Beginners' Resource Centre
That information explains about the different chemicals and bacteria that make up the "eco-system" of a healthy tank, and how long these bacteria take to become established. It also outlines the equipment you need to give your fish the best start.
Once you understand how the cycle works (and why it has to take as long as it does), you can explore ways of cutting that time down until you can add fish.
Here are a few menthods:
1) Fish-in cycle - similar to your goldfish method. You add a tiny number of fish, test the water daily and change as much water as necessary to keep ammonia and nitrites at zero. After a month or so you can add more fish. PROS: you can add a tiny number of fish right away. CONS: requires you to test the water daily, do large water changes (50%-80%) daily and potentially causes great harm to the fish, limits you to "hardy" fish initially (which will still get hurt/sick/die, but are
in theory a little tougher than others), still takes around a month.
2) Mature media (i.e. ceramic rings or sponges from an established tank). PROS: you can add a few fish right away and you speed up the biological development of your own filter as the mature media already has established bacteria. CONS: easy to misjudge how many fish to add, still requires you to test water to make sure it is safe and you have to add fish within a few hours of adding the media or the bacteria will die.
3) Nitra-zorb/zeolite - this stuff chemically removes ammonia. PROS: removes ammonia artificially so you can add a few fish right away, can still allow you to develop you biological filter
as long as the biological media (sponges/ceramic rings/etc) is placed before the chemical media in the filter (i.e. so the water hits the biological media first, giving the bacteria "first dibs" on the ammonia. CONS: expensive, hard to find, doesn't last very long, you still need to test every few days at least.
4) Good old fishless cycle - uses ammonia to replicate fish waste and develop your biological filter in a sure-fire and non-harmful way. PROS: does NOT harm fish, allows you to add 50%-80% of your stock as soon as the cycle is complete (as long as you have the appropriate filter for the tank and don't overstock according to the rough inch-per-gallon rule), gives you time to carefully consider stocking levels and helps your understanding of the biological cycle. CONS: requires you to add ammonia and test every day, takes about a month (which is how long the bacteria take to grow from scratch, whichever method you use).
5) "bacteria in a bottle" products. PROS:
If they work, they'll give your cycle a
tiny nudge CONS: These tend to be advertised as allowing you to add your stock right away and don't give you sensible instructions on how to do that safely, and they
don't tend to work. Wasted money, IMO.
Reasons why your previous method isn't so great:
1) It harms the goldfish.
2) 5-7 days isn't even enough time for the first set of vital nitrifying bacteria to establish, let alone the second set.
3) If you don't have enough bacteria, your intended stock will be affected by "New Tank Syndrome"
whether they show symptoms or not.
4) You say you never had any problems - did you test your water daily for ammonia and nitrites? It'd be a miracle if these were zero throught the entire process. The biological filter simply does not establish that fast.
Bigger tanks don't take more downtime. The cycle will take around a month in a 5 litre tank or a 5,000 litre tank.
Good luck!
