Age and lack of experience is no excuse. These plecs should grow pretty rapidly so in the time you have had it he should have reached somewhere around 8-10" - this lack of growth is most likely due to tank size, lack of adequate filtration and overcrowding.
The 'signs of stress' above are very very general and will not always apply. You really need to address the space you have this plec in or he will end up with permanent internal damage.
Immediate things you can do are increasing your water changes to compensate for the poor filtration, I'd do 10% daily and 30% twice weekly as a bare minimum until you can get a more powerful filter.
In the slightly longer term you seriously need to either change your stocking (take the unsuitable fish back to the LFS or give them away/sell them to someone who can house them properly) or upgrade your tank significantly size wise. Do some research on the size tank a plec like this needs - not to mention the other fish!
I know it can be difficult and there is a lot of conflicting advice but you did ask...
The 'signs of stress' above are very very general and will not always apply. You really need to address the space you have this plec in or he will end up with permanent internal damage.
Immediate things you can do are increasing your water changes to compensate for the poor filtration, I'd do 10% daily and 30% twice weekly as a bare minimum until you can get a more powerful filter.
In the slightly longer term you seriously need to either change your stocking (take the unsuitable fish back to the LFS or give them away/sell them to someone who can house them properly) or upgrade your tank significantly size wise. Do some research on the size tank a plec like this needs - not to mention the other fish!
I know it can be difficult and there is a lot of conflicting advice but you did ask...