It is amazing how so called "starter" fish are very difficult to keep well. Swordtails need much space and excellent water quality. Barbs need excellent oxygen content in the water, tetra's can not handle large fluctuations in water chemister, etc. The so called "bread and butter" fish that the hobby was started on are incredibly difficult to do well over many years. This is a lesson that has taken 20 years for me to understand, but is still difficult.