Just do a minimum of 50% water changes per week. Feed a varied diet; stick with the spirulina sticks, use a good quality pellet, frozen bloodworm, crickets and meal worms for live food but no live fish, you can also try some raw prawn chopped up quite small. The varied diet will ensure excellent nutrition and that nothing is missed in it's diet. The spirulina and a good pellet will enhance it's colour quite a lot but the rest will help, the spirulina will aid in it's digestion and keep the little guys guts clean and regular.
More space will also help, get the largest tank you can fit and afford to buy, please do not get the minimum unless that is all you can afford or fit. Remember to look ebay and craigslist for cheap but large tanks.
A 75g should really be your minimum, 12"+ GT's are rare probably because many die before they hit full size or are sold on before they reach it.
Keep an eye on it's fins, if they do not start to heal then it is a good bet that something is nipping, probably the African seeing how mangled a BP's jaw is. Stress will not help so check for chasing as they are easily stressed when young.
Even poor quality GT's bought from places like petsmart look good, 90% of the GT's shown by the Americans on this site are places like Petsmart. I think your one just needs some TLC. Give it time.