Hello and welcome to TFK
When you set up the tank did you do anything to start the cycling process? For the cycling process to start there needs to be introduction of ammonia. Here is a following article on bacteria in the tank which will help to explain more about the cycling process and what happens in your tank http://www.tropicalfishkeeping.com/f...91/#post757735
Also do you have any live plants in this tank or are you planning on adding live plants to this tank. If the tank is planted heavily enough it will help with the cycling process.
As mentioned by others both tiger barb and serpae tetra are nippy fish, especially if kept in small groups. It is recommended that both be kept in groups of at least 8 to help reduce fin nipping. As for the compatibiltiy of these two fish together I am not sure about that myself. Hopefully someone with more experience with these fish will be able to give you more advice.
As for bottom feeders a few options that you can explore are different types of corys that will work, or even some different catfish would work. Do you know what the hardness of your water is? You can find out this information from your local water people, most have this information posted online. If you can not find the information online then you can call your local water people and ask them for the gh (general hardness) and kh ( carbon hardness). Some fish are more particular than others in water conditions, and this information would be good to know in being able to choose a fish that is suitable for your water.
For more information on different type of fish, if you have not found it already there is a section on fish profiles. The link can be found at the top of the page second tab from the left. Also you can look at different fish profiles by clicking on the highlighted name like tiger barb. This is a good place to start in trying to figure out what may work with your setup along with finding something that may interest you in keeping.
If you have not already gotten a test kit for checking your ammonia, nitrites and nitrates I would highly recommend in investing in one. You wil want to get a liquid test kit, like API master test kit, which many of the members here use. The liquid test kits are more realiable and accurate than the stick tests, along with that they are a better value for your money.