need help plz!!

Question
Hey my name is travis and im fairly new in fish keeping. I have a 20 gallon tank with 1 dwarf gourami,1pleco,2 painted glass tetras,1 serpae tetra,1pristela tetra,2 neon tetras, 2 glo-light tetras, 1 angel and 1 snail.temp is btw 79-80f.aquascape gravel,fake trees. I am wondering if my tan is over populated. i have also been having alot of deaths for no apparent reason. my serpae tetra is agressive towards every fish in the tank. I have recently recognized that my neon tetras have nearly no tail left. I forgot to mention i have one female guppy in there also i have a 5 gallon tank with 10 fry i was wondering if i would be able to put a few guppys in the 20gallon when they mature a bit? Thank you very much and sorry for the long post. Im in desperate need for some help. my ammonia levels are 0.25 and i do frequent water changes once a week.(gravel cleaner)Any help i can get would be greatly appreciated thanks alot. Travis

Answer
Hi Travis:  your ammonia is a bit high... take all of the ornaments out and clean the gravel really good... change 30% of the water. You may have to find another home for your aggressive tetra.  You can put the guppies in the tank when they are about 3 months old.  You should try to add some real plants in there as well and also some aquarium salt... read the instructions... Keep me posted... dave