IMO - you are feeding to much. There is nothing like satiation for fish more than for a very short time. All all carnivorous and omnivorous fish are designed to eat as as much as possible when there is adequate food - it will maybe not be around all the time. Because that they never know when food will be present again - they are designed eat and to store nutrients reserves just for bad times - eat and store as much as possible when it is possible is the melody. They store it as fat and in also as proteins (if they exercise enough - read swim a lot in strong currents). This reserves is meant to be used rather fast (in temperate species - within a year) - for tropical and subtropical species it will be used in even shorter period of time or for constant breeding.
Many omnivorous fish (and some predatory species too) feed on low nutrient prey - it means they have to eat very often in order to get the nutrient they need
For mostly herbivorous species - there is another problem they have to solve - the food itself is very nutrient rich but the nutrients is protected by cellulose walls. Cellulose is a compound that is very difficult to digest - especially for fish that all seems to lack the enzyme Cellulase. They try to solve this through a very long digestive track - very much longer than the length of the digestive track of carnivorous and omnivorous fish. with a good population of bacteria (some can produce cellulase) and time - at least a part of the nutrients will be available for the fish but most of it will only be pretreated and come out as very much poop. However this poop is a very good food sources for other organism as shrimp, hermits and other detritus eating animals because much of the breakdown process of the cellulose walls is already been done by the fish.
This means that carnivorous and omnivorous fish eat and eat if food is available. If they in nature feed on low nutrient prey - they eat all the time they could too. If they eat too much (or too nutrient rich) food they build up reserves (read fat) and if they get food all the time (or too nutrient rich food) and not get periods there they use the reserves - they get problem. They get to large and often malformed
For mostly herbivorous species - there is another problem too. The long digestive tract is designed for process indigestible food and at least get out some nutrients. They are also designed to eat all the time because they maybe need up to 8 times more raw-food than carnivorous and omnivorous species in order to extract the same amount nutrients for their growth.
If it will be bad to give our carnivorous and omnivorous fish to much, to often and to nutrient rich food - it will be a disaster to give mostly herbivorous species the same. Diseases caused by hexamite like organisms is more common than you can believe among herbivorous species and caused by too digestible food (for the digestive system)
Someone says - but look at fish farms - they use this type of high quality digestive food without problems.
That´s right and the fish farm industry is very good in produce food there the meat production will be optimized and because of the genetics of fish you can just push highly digestive predatory food through the short digestive tract of carnivorous and omnivorous fish and get a 1 kg fish after 1/10 of the time it takes in the nature. but the lifespan of farmed fish is normally not longer than 2 years - after that its time for us to eat them.
But our fish - they should not live for 1 or 2 years - they should live for many more years.
I do not say that´s wrong to feed with highly digestive dry food but do not feed too much and be very careful with fish that are mostly herbivorous. If you ask the fish - they are like children with that exception that children can eat how much candy you give them instead of food.
A yellow tang seems to be between 8 and 10 years old when it is around 150 mm total length (from snout to end of caudal fin) in nature. Length depends on sexes. A female stop around 150 mm and a male around 170 mm. They can be up to 40 years in nature but most growth is the first 10 years.
See here In that article - it is a very good chart of yearly grow rate in nature . in short - 0-1 year around 32 mm - 1-2 year around 25 mm, 2-3 year around 18 mm, 3-4 year around 14 mm, 4-5 year 11 mm, 5-6 year 8 mm and so on. It means that a 100 mm yellow tang (10 cm) is around 5 years old and if it is in your aquarium it should grow 8 mm the following year if it was in nature
I hope this answer some of your concerns and questions.
But if it works for you - do as you do but hopefully will my post give some new ways of approaching the question.
Sincerely Lasse
