The tinfoil barbs are highly aggressive and territorial, could you rehome them and repalce with a much nore peaceful fish.
The LAST thing the OP should be thinking about right now is replacing any fish. Aside from that tinfoil barbs are very peaceful fish, their size is the issue, NOT aggression. prakash8820, Please don't add any new fish into the tank.
prakash8820, it looks like as if some very bad advice has been given by the person at the store and I'm glad you've come to this forum. Unfortunately, it looks like the advise given by the person at the store has led to you missing some of the most important (and most expensive) parts of the fish tank.
Firstly, an undergravel filter alone is not sufficient to filter your tank and this will make your problems worse. You should invest in an internal filter, the sort that attaches to the side of the tank. A good filter for your tank would be the Fluval 3 or 4 plus.
Secondly, I'm guessing as you don't know what additives to use the water in the tank has come straight from the tap. Tap water has lots of additives, namely chlorine, that are harmful for the fish. Therefore, a dechlorinator (found in a bottle at your store) should be added in the appropriate amount to remove the chlorine and make the water safer for fish.
24 hours is not an adequate amount of time to leave the tank, and it shouldn't be "inactive". To make the water safe for fish, you should go through a process called cycling, which, when done properly, can take less than a month to complete. It'll take too long for me to explain the whole process of the cycle, and as you already have fish in there you'll have to do what we call a "fish-in" cycle. It's not ideal but it'll have to do for now, and I suggest you read the appropriate title (fish cycle) in this web page:
Cycling a Tank
The science isn't all there but it's probably the simplest explanation you'll find, and the easiest to follow.
To do a cycle you'll need to test your water for ammonia, nitrite, and nitrate. The best way of doing this is using a liquid dropper test, such as the API master test kit (my personal favourite). The instructions are very helpful in using the product properly.
Finally, you don't need to take out quite that much water. A simple 25% water change a week is completely adequate.
If you could possibly post the exact dimensions of your tank (height, length, depth/width) I can quickly work out the volume of your tank.
I hope this has helped and isn't too much of an information overload - but this has happened to almost everyone new to fishkeeping. From now on, I'd strongly advise you ask any questions on this forum rather than in the pet store. On here you'll find a large amount of experts who have been in the hobby for years and will give you honest advise based on what's best for you and your fish.
