their in lies the problem i think w/ mixing knifes and eels. Their needs are so similar they're fighting over the same "territory" Fire eels in particular seem to avoid eating other fish. Mine has the option of eating feeder guppies in his tank, and they are always available to him. ( the tank seems to be my new guppy breeding tank ) I'm sure he probablly eats some of the fry but my guppy population is steadily increasing. Time to feed the clawed frogs again! My best tank-mate suggestions would be larger tetra species of the docile sort, or possibly some rainbow fish (mid-and topwater fish). Anything that won't compete food or space wise. my guy is about 12" now at haveing him/her about 1 1/2 month. My hand goes in the tank and a lil' nose pokes out immediately. he won't eat out of my hand yet but he is comeing out now when I drop the worm in an open area. And will actively scavenge frozen bloodworms with me watching.
They also seem to be one of those hit or miss fish

By that I mean I purchased 2, both in excellent visual condition. and Lost one of them about a few days ago after having some issues with my tank. I also lost my pleco at the time. But my remaining guy is fit as a fiddle and eating heartily. They where very timid and shy when I first brought them home. I plan on replaceing the one i lost with 2 new ones

as my remaining one has me hooked. I've ahd TT, and Spiney eels in the past and always wanted these guys. The first thing i'll do different is add alot more hidieng spots to the tank. PVC (sharpe edges smoothed out!) and lots of driftwood. I also am changeing my current gravel which is relatively fine. to a course sand-like gravel ( the gravel is smooth, just larger granuals.) Stress seems to effect these guys much like it can some of the rarer loaches and fish like Cardinal tetras.
If I where to mix this fish personally, I'de place the eel in the tank first, make sure it is eating well, and comfortable with the tank. Then toss in some docile but fast mid-top water fish. I've seen picture of these eels in community planted tanks. One of the most important things imho to really think about with this fish is it's potential size 3' is not a small fish. and keeping your fish in his or her tank. just like TT's and Spiney's tehse guys liek to jump, though so far mine has behaved hismelf it seems. But I have tank locked down tight.
