Dear Joe-Turbo,
One Black Moor really, really, really, really needs at least 20 gallons (US) to keep healthy. Please consider using a plastic 20 gallons bin which can be obtained more reasonably as an interim home for the Black Moor as you save up for a proper 20+ gallon aquarium. These bins are usually sold in the house goods sections of department stores and storage shops. Rinse out well before use and don't use the top.
Moors also require as do all goldfish 10 times the gallon size filtration per hour. So you will need to up your filter to one that turns the water over 200 gallons per hour. (Goldfish have a simple digestive system and therefore produce more waste more often.) Many Goldie people do at least 75% water changes weekly to stay on top of the nitrates.
Without a 20 gallon home for the Moor, you will have to try daily testing of the water--even ammonia can cause illness, etc.--and if necessary to keep ammonia, nitrite at 0, daily water changes with de-clorinated water.
Chris+