First off, well done on researching and asking everyone for advice on how to set up and stock your 200l tank, so many folks just jump in head first and make mistakes right off the bat, good going
Now first, set up your tank exactly how you would like it, lots of live plants is excellent as this brings in so many benefits to the tank. Start off with easy to care for plants as other have already mentioned as well as floating plants if you wish to do so.
Then you need to cycle your tank, there are in fact several ways of doing this.
You mentioned you have some spare sponge media in your established betta tank, if this is correct then this is a nice addition to seed your tank and filter with, if not, then you can purchase a bottle Tetra Safe Start or Dr Tim's One and Only Nitrifying Bacteria (both easily obtained online), both of those products have the correct bacterias needed to get the cycle off to a good start.
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If you are unable to get bottles of bacterias or use your sponge media, then not to worry at all, your plants for a start will bring in some bacteria to start you off, not as much but its fine, just takes a little longer to get these bacteria to grow a large enough colony to start making an impact on ammonia and eventually nitrite.
Ammonia dosages is essential for cycling a tank, you can get some online such as Dr Tim's Ammonium Chloride which is fine to use for cycling aquariums and will not impact on plants. I've used ammonia on cycling tanks many times and the plants have always been fine.
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Do be sure to follow the Fishless Cycle Article exactly -
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
www.fishforums.net
And also do note that you must dose ammonia 3ppm, no more than this as anything over 5ppm ammonia has a tendacy to crash or stall the cycling process, then you would need to start the cycle all over again by doing a 100% water change, so do be careful with dosages as it's so easy to make a mistake and overdose, I've done this myself!
You can use the Forum calculator to work out exactly how much ammonia you need for your tank, the Fishless Cycle Ammonia Required dosage calculator is at the bottom of the page.
Again, read and follow that article on Fishless Cycling that both I and AdoraBelle have linked, I would print it out actually then you can read while you do the dosing or while next to tank etc, I found it much easier this way.
Cycling pro!! hah
Nope, just done a few of these cycles is all and have the general idea on how to do a cycle, much harder to explain to other folks how to do the whole process. Hence why we have a fishless cycle article.
That article is quite hard to follow at first, but reading it again and again helps to build up an idea of what you need to do, a lot of information to take in at first, but once you start and within a week or two, it kinda clicks and aha, now I understand a bit more.
Took me and TTA a long time to write up those articles as well as the Fish in Cycle articles as well.
Well, we did the best we could. Pics a bit dated now as that was written years ago but it has served its purpose very well.
There is one other method of cycling the tank and that's by having very densely fast growing plants in the tank and watching the tank parameters closely and doing water changes when required, a number of members have used this method but I have never done this personally so am not the best to ask about this method.