New aquarium setup with live plants

Question
I have an old aquarium 20*12*15 with plastic plants, i wanted to move for a more realistic and bigger tank so i went to my pet shop and discussed for a tank with live plants, he has suggested me a 36*12*15 with open top,and he assures there will not be a need for co2 process. He has added a Top filter, 20kgs of sand and 8kgs of fertilizer.As for the plants since he had to bring it from the nursery i wasnt able to choose yet. I have already placed my oredr and getting my tank set up this weekend. As for fishes i currently have 4pairs of zebra danios, 1pair redy eye tetra,3 firetail guppies and 1pair of red molly. Please let me know if i can have any changes to the size of my tank, do i need a co2 process or can i do without it? will my fishes jump in a open tank? I plan to add tetras, how many can my tank accomodate after decorating my tank. what fishes can be added to the exsisting community? Since i have had mollies jump in my bowl should i remove it?

Answer
Hi Karthik, sorry this too so long. I think having an open top is a horrible idea. Many fish, and some of the fish you have are jumpers, and they will jump. You don't want to come home and find one of your fishes dead on the floor. I would recommend that you get a tank with a top!
Also get a co2 process, since you wont be having the top open. Its a good idea, so that your plants wont die either. The amount of fish you have is a good amount for you tank. Adding tetras I would say is about it. You don't want to kill the fish by adding to many fish. So just remember to keep the amount of fish limited. If you put a top over the tank you wont have to worry about any mollies jumping around.

Alejandra