A single Royal Gramma can be kept in fairly small aquariums from 30 gallons upwards. The standard advice is to only keep one RG per tank, however it is possible to keep a pair if done correctly. It is unwise to introduce more than one into a tank smaller than about 50 gallons. Royal Grammas are territorial fish, and although their open mouth display doesn’t amount to much with other fish, it can progress to injury or death between two RG’s.
If you have the correct tank size to accommodate two and you would like to do so without serious injury occurring you have a few options available:
Option 1: Your LFS may already have inadvertently paired a couple of RG’s by adding them into the same holding tank as juveniles. This is how I purchased my pair.
Option 2: Purchase two juvenile RG’s both under 1” in size. Both will be females and one will become the dominant male. You may experience some scrapping to ascertain dominance but this should not last more than a couple of weeks. This would be best done in a quarantine tank where you could treat them with API Melafix to aid recovery if required as they can nip at fins. Always ensure there are adequate hiding spots in a QT – see this topic on how to quarantine fish.
Option 3: Purchase an RG over 1” in size that appears to me a male – see here for sexing. Then add at the same time a juvenile under 1” in size. The Juvenile should accept the dominance of the male and remain female.
Do NOT under any circumstances add Royal Grammas at separate times. One RG will have asserted its dominance in the tank already and will not take kindly to another arrival.
Royal Grammas have also been kept safely together in a ‘harem’ consisting of a male and several females. This emulates how RG’s live naturally. Again, the number one take away point is that they are always added together. A harem should not be explored in a tank under 100 gallons.