I came to conclusion that I am ready to buy 20 Gallon long tank. Now need more help above question
i would go for 10 zebra danios or platies once the tank is ready as these fish are hardy and help bring on the filltraison system in the tank .
I agree with danios and platies being among the very best of the hardy fish that can be introduced to a new tank, but I respectfully disagree with the idea of having a beginner use them to cycle the filter. One of the great things about this "New to the Hobby" forum is that it provides a sort of guide center to help beginners be able to get through their first (and hopefully only, since thereafter you can just clone your bacteria) fishless cycle.
While its entirely possible for a beginner to Fish-In Cycle a filter, just like an experienced aquarist, it often doesn't go well and the mistakes come at the expense of the fish, not to mention that it can be a huge hassle for the beginner doing frequent large water changes. In contrast, the labor involved in a fishless cycle is next to nothing, even though the patience part is difficult.
My personal preference, if I were milindsaraswala, would be to introduce a small to medium number of inexpensive plants (they may get covered with algae) and to begin a fishless cycle in the 20 gallon tank. The article to read is our pinned one at the top of the forum by rdd1952 that tells about the Add&Wait method of fishless cycling. If at all possible, some mature media should be sought out from another TFF member or a LFS or whereever you can find any. You will have to get a good liquid-reagent based test kit and find the right kind of pure household ammonia, which can be difficult. The plants sometimes carry a few bacteria in with them and so might possibly help seed the fishless cycle a little although not anywhere near the degree to which a piece of mature media added to the filter will. Good luck and keep asking here milind~
~~waterdrop~~