There could be loads of reasons, but I will try and help. Firstly I would suggest a water change, I know you said the water is fine, but see if it perks the fish up any. How is it breathing? does it have rapid gill movements or is it relaxed? If a water change helps, monitor it over the next couple of days, you may need to add more surface aggitation, or an airstone.
In my experience with severums even small ones can be little bullies, it maybe you have 3 males and one female or all male, golds are harder to sex unfortunately. A subordinate of the group will spend time stressed out hiding, speaking of which how much cover do you have in the tank? I would say add some large plastic plants and lots of wood to break up as much eye contact as possible, if the one that is not doing so well then starts to pick up once he/she has a hiding spot then you know that is the problem.
How long have you had them? it could be that it is just sick. Fish will go a while without eating so no need to overly worry yet, check its poo is normal, if its white and stringy it could be a worm problem and you would need to treat them all with an antiparasitic medication, also check for any signs of red worms hanging from its anus camallanus, these are nasty little blighters, these can affect fish eating. If it is camallanus worms, you need a medication called sera nemotol, its the most affective against these worms.
Or the fish could be suffering a bacterial problem, daily water changes can help, if the fish improves continue water daily water changes then gradually reduce them back to the normal weekly water change. If it doesnt improve with daily water changes then a boost with an antibacterial medication maybe needed.
Stress could also be a factor, being moved, new environment, note getting on with its tank mates all could be a factor. To get a sick fish to have an interest in food you can try soaking food in garlic (this is also has natural antibacterial properties) or you can try a product called Vitazin, this has added vitamins, you just soak the food in it, I have had great success using this with some very sick fish.
I am assuming they are still quite small as you state in your opening post that you are growing them out. Hope this helps.