Originally Posted by Cassie_KY
I have not kept any of the fish you are currently wanting. However, I have my heart set on a black ghost knife for my 125 and have done some considerable research on them. It would need some more vegetation and cover to feel comfortable in the tank. He and the pleco might argue if the pleco tries to share the knife's hidey hole with him. The barbs may be a snack for the knife down the road as knifes eat smaller fish if they are not fed very well by their owners. *Sigh* I love this fish!!
A ghost knife would be very temporary in a 55 gallon tank. They get quite large and like to spend time hanging vertical... also are very shy and can be fussy eaters. Ghost knives require warm temps (80 - 82) large tanks, good filtration, and good maintenance. Ghost knives are a scaleless fish and very sensitive to water quality and aggressive tank mates. They are also a member of the electric fishes, using electrical shocks to stun their prey and in self defense. Minimum tank size for 1 black ghost knife is about 75 - 90 gallons.
Pacu- they get three feet in diameter. If you don't want another tank, don't get one of those. If you like that look of a fish, silver dollars look similar and only get about four-six inches (can reach ten, but not often) and many hang out around five inches. They are schooling fish and are happier with friends. The males even have some red coloration and look alot like red-bellied pacu's.
Pacu get to 5 feet, and we have a whole tank full of them at that size at our local zoo. I spent a lot of time sending people to see them before I would agree to sell them a juvenile, and it was with that I convinced my boss to stop selling them. Pacu are not appropriate as "pets" and don't belong in a home aquarium.
The others, I honestly don't know a thing on them. Anyone else hanging out keep those kind of guys??
The dinosaur bichir is actually another name for Polypterus senegalus, and they also grow incredibly fast, can be quite aggressive and average 16 - 20 inches long. They would feed on your barbs and danios both, and anything else that would fit in its mouth. This also would require a minimum of about 125 gallons and would be too aggressive to keep with a ghost knife.
The peacock eel is another tank buster of sorts... average of about 12 inches long full grown. Peacock eels would need at least 90 gallons if meant to be kept with other fishes. Surface area (shape of the tank) is very important and also a sand substrate. Peacock eels spend most of their time burried in the sand, gravel is too harsh and causes injury quickly. Also a scaleless fish, water quality is of utmost important, and they are prone to fingal infections. An adult peacock eel will hunt anything that will fit into its mouth, and they are incredbily fast and tend to jump a lot.
Freshwater needlefish grow to about 16 inches, and need a very long tank. Anything less than 125 - 150 gallons is a no no, and that is pretty early on. These fish grow fast, and by the time they reach full grown would make lunch of the danios and barbs. These guys are big waste producers so good circulation and plenty of filtration, and lots of water changes is a must.
I hope this helps you to sort things out. As fun as all of your choices would be, they are not all compatible and none of them is suited to spend any real amount of time in a 55 gallon tank.
I get a feel for the eel-like appearance and movement that you seem to be attracted to. Have you considered that in order to do that, you'd likely have to change the gravel to sand? Silica sand is the best to use, and is relatively cheap... just can be tricky to keep clean without clogging drains/plumbing.
IF you decide to try a sand substrate, here is one option for you to consider: http://www.loaches.com/species-index...antella-maassi
Also, kuhli loaches are a lot of fun and get to about 5 inches full grown, so suitable for a 55 gallon tank. With the other fish you have now I would just be sure to provide a lot of hiding places down low for the loaches.
Another fish to search for are the halfbeaks. You will get the same shape as the needlefish (mostly) without the size and predatory habits. Some species of halfbeaks get to 5 inches, so they aren't tiny fish, but much easier to work with in a 55 gallon instead of a needlefish.
Before I go I also wanted to warn about salt in the tank with the types of fishes you are looking at keeping. Many scaleless freshwater fishes will not do well with salted water, and it should be used for medication purposes and sparingly if ever. The "salt treatments" using just basic aquarium salt is not a good thing for any of the fishes long term. It is primarily sodium, which builds up and also can cause long term organ function issues and permanent damage. If salt is needed for a freshwater aquarium, it is best to use a marine salt mix, which has much more mineral content in it, and lower sodium levels.