We have two turquoise severums. Both about 8 months old, 5"... live mostly alone in a 75 gallon (US) tank. One's definitely a male... the other is hard to say. The one I'm sure is a male is the more "territorial", and his coloring is very bold in the classic green severum colors, nice greens, iridescent blues, and vivid red pelvic and ventral fins.
The not-so-bold one has always been a lot more pale, occasionally showing off vivid dark vertical stripes. Still happy and healthy, though. Always swimming up to the top of the tank for dinner when I come home from work.
I usually do 50% water changes every 10 (or so) days, refilling with conditioned water. After the last water change, the paler Severum has gone from pale... to almost solid black. Health seems fine mostly, but a little more skittish in the past couple days, appetite is great (still steals and swims around with our pleco's daily giant algae wafer).
I think I *have* noticed a change in the dynamic between our two sev's... could it be a passing of the "alpha" torch? Perhaps a maturity thing? Has anyone else had their green severum go from pale to dark, dark green? Our other severum is still showing his usual vibrant colors.
Water parameters (pH, hardness, alkalinity, nitrites, ammonia) are all fine. Nitrate is at 20ppm. Only thing different that I've done is put the aquarium light on a timer. About 12 hours on and 12 hours off, hoping to normalize their day from my semi erratic day. :roll:
i also have a pair of greens. younger and smaller than yours but depending on mood, time of day and if theyre eating, they go anywhere from almost black to lighter with very bold stripes to goldish yellow. its funny, i have noticed that its consistantly morning - stripes, during day - lighter yellowish gold, evening - almost black.
i think my larger/dominant is a male and the smaller lighter colored one is female. im in the same boat as you, trying to figure out which is what sex. they sure are interesting to watch and try to figure out. please keep us posted to any new discoveries.
what exactly do you keep them with in the 75? im getting ready to move my guys to a 75 and im not sure what else i want to stock.