I have a 35.5litre tank that currently houses 4 male guppies ( who all get along fine and dont pick on each other) 4 male swordtails ( who are the same size as the guppies i think they are fully grown cos the are a year old now they were the fry from one of my female swordtail they just havnt grown properly) and 5 algae shrimp. Tank still looks a bit bare asa i used to have 12 guppies in there as well as the swordtails. Could i add any thing else to the tank i would love some tetras. The tank has been set up for over a year now and has an under gravel filter and a fluval one plus filter. Never had any problem with nitrite rising in the last 6 months. NITRITE currently 0, AMONNIA is also 0. Thanks 