I do not agree with Byron re rinsing the old substrate. What is in it is plant food and I work hard to get and keep in there in my planted tanks. It may also contain bacteria we want. Google "mulm"
So how would I do what you are about to?
First Stage
1. Set up the new tank where it will go, be sure it is clean.
2. Bigger tank = more substrate. I add the new substrate first because I want the current substrate to be the surface layer. It holds the good bacteria which needs oxygen to function. I rinse new substrate and put it in wet.
3. I set up a few buckets/containers which I will fill with new dechlored water at a similar temp. to the 5 gal.
4. I remove the plants to a container, I move the hardscape/decor to one as well. Then I remove the fish to a container. The fish must be in their own container but the rest can all be together.
5. I unplug the filtration and heating.
5. I lower the water level in the 5 in order to get the substrate out. This can be done via a siphon or a clean spatula. I then spread the old substrate across the top of the new.
6. I immediately moisten the substrate now in the new tank. Enough water so it is covered.
7. I take a few minute break.... Before setting up the new tank.
Set-up
1. Because I do not want the hardscape/decor to dry out I add more dechlored water to the new tank. It should be enough to cover most of the hardscape/decor. I am guessing I fill to about 1/3 the depth for this step. If I am putting in a heater horizonatally near the bottom, I put it un place but do not plug it in yet.
2. I add the shorter foreground plants.
3. I fill the tank more to about 2/3 full.
4. I plant the mid-ground plants.
5. I fill the tank close to its final level.
6. I put the filtration in place, but do not run it yet.
7. I plant the tall background plants.
8. I turn on the filtration and heating.
9. If I am happy with how it all looks and works, I add the fish to their new home.
Other considerations before starting.
- The filtration does not have to be removed from the 5 until it is time to put it onto the new tank. If you are not going to use the filter on the 5 on the new tank, then the media in the existing filter must be preserved. By this I mean it must be kept wet.
- If getting a new filter or heater for the bigger tank, rinse these well before using them.
- If the old filter will not be used, then try to put its media into the new filter along with the additional media it should hold. Failing that , rinse out old media in the new tank water. If you do this, then before adding the fish, let the new filter clean the dirtiness you just rinsed into the water. That stuff holds bacteria that your want to end up in the new filter's media. Also, place the old media in the new tank for about a week or two and then remove it.
- If you will use the current filter plus a new one. It helps to put a bit of the old media into the new filter and then to replace what was removed from the old filter with new some new media. This suggestion assumes you will be adding more fish once the new tank is up and running OK.
- If doing all the moving etc. before adding the fish to the new tank takes a lot more time than expected, you will need to be sure the water the fish are parked in stays warm and oxygenated. I often scoop out some water and replace it with new warmer water. This also serves to oxygenate the fish water. If this were a bigger scale project I would be using a rubbermaid container with a heater and airstone in it. For everything but the fish.
- When you are ready to add new fish, you will have to do so gradually to insure you do not have cycling issues.
Live plants are great filters. The more of these one has in a tank, the fewer bacteria there will be. But there will still be some amount of them no matter how many plants one has. In a well planted tank most filters serve as mechnaical filtration and as oxygenators and circulation aids. But they will still have some bacteria in them.
Basically, if you follow the above your new tank should be safe for your existing fish. If it makes you feel better, you can test for ammonia a few times over the next week. However, as noted more water means ammonia will take more time to build up. The odds are very good that the esisting bacteri will multiply sooner than any ammonia or nitrite can build up so it shows in a test. But it never hurts to be sure.