Red Belly Pacu sick?

Question
About 6 months ago I rescued a Red Belly Pacu that is about 16 inches long.  I have him in a 125 gallon tank. The previous owner said that he is probably 2 or 3 years old and his old diet consisted mostly of hot dogs. yes that is not a typo...hot dogs. Was feeding the pacu hotdogs really bad for it? Since i have had him or her I have feed the pacu vegetables and large pellet food.  I have searched the Internet for info  on caring properly for a Pacu of this size.  I have not had much luck.  My newest concern is the pacu's head.  Just above the left eye the head seems to be sinking in.  Is this due to an unhealthy diet?  also what is proper tank care?  i assume due to his size tank care will be different.
thanks

Answer
Good afternoon Gray, thank you for your question.

I've actually heard of people feeding their big fishes hot dogs before. Pardon the stereotype, but it seems fairly common among teenage boys (either at heart...or actually teenagers) to feed their piranha-looking fish such as silver dollars and pacus, such meaty fare. Feeding their herbivores roast beef, chicken, turkey - I've heard it all. It's a wonder it survived. Hats off to you for your noble rescue act, I wish there were more people like you out there!

I've read (and experienced) many cases of fishes, seemingly fine in spite of years of neglect and/or the wrong care, living for years this way... Then, after you do all the right things, symptoms appear and persist. It sounds like your pacu has what is commonly called hole in the head or HLLE, Head and Lateral Line Disease. The causes are hard to pinpoint, but poor diet seems to be a big contributor.

It may hearten you to read the article below:
http://www.aquarticles.com/articles/breeding/Marshall_Pacu.html
With a few paragraphs about rescued pacus towards the end.

There is lots of available about their care online, here's a few websites I recommend you try first:

http://badmanstropicalfish.com/profiles/profile86.html
http://www.aquatic-hobbyist.com/profiles/freshwater/chariciforms/redbellypacu.ht...
http://www.wetwebmedia.com/FWSubWebIndex/serrasalmfaqs.htm

Here are few suggestions as to what to do to nurse this pacu back to health. This could apply to most any rescue fish.

1. Provide a nutritious and varied diet,
2. Very clean water, changed often regardless of parameters,
3. No aggressive tankmates in the tank now,
4. Or plans to incorporate future ones. Pacus themselves tend to become more aggressive as they get older.

1. I recommend you purchase a vitamin supplement such as VitaChem. Herbivore pellets or spirulina flake that you feed should be soaked in this solution 4-6 times a week. This will supply much needed Vitamin C to help with the possible HLLE and fill in any nutritional gaps. It's like a multivitamin for fish! Here are other feeding recommendations for this species:
http://fish.mongabay.com/species/Piaractus_brachypomum.html

And in general for pet fish:
http://fish.mongabay.com/food.htm

2. If at all possible, perform twice weekly water changes. As much as 20% each time, if you can manage this, would be very beneficial. A Python No Spill system might prove to be your best friend. The reason for this water change schedule is that metabolites accumulate in the water, which cannot be measured in the usual way (nitrites, nitrates) but prevent growth and development, and presumably, healing. It is best to flush these out with frequent partial water changes as often as possible, especially in the case of a sick or recovering fish or fishes. Keep your filter media clean and your water well aerated. Pacus are known to be skittish, so keep this in mind when performing maintenance. It is not at all uncommon to read of pacus who have injured themselves by launching themselves against the hood or tank sides.  

3. This is a given. Fishes too small will be eaten, but this peaceful fish cannot defend itself very well against any large predacious fish. Certain catfish fall into this category, and I wouldn't trust the sucker on a Chinese algae eater, either.

4. It sounds like this is going to be a species tank for your pacus. Include some driftwood to make them feel at home, and consider installing a heater guard as these lugs (affectionate term!) have been known to crack theirs.

Adding some salt will restore electrolytes and relieve stress, try it and see. A light teaspoon of kosher salt or aquarium salt per 10 gallons would be my recommendation.

I hope that helps! Feel free to write back if you need clarification, and please let me know if I can be of service again. Again, thank you for your benevolence towards these fish!

Nicole