The number one advice I would recommend you follow...
Research, research, research.
The article section of this site has lots of good articles on how to cycle a tank, and how to test your water. Once you have a solid grasp on basic fish-keeping, then you can move on to basic husbandry of the species you've tossed together.
First off, Black Ghost Knifefish are one of the most challenging fish to keep alive long term. They're nocturnal, extremely shy, and easily bullied into starvation by the larger clown knife and the bichir. The vast majority of them die within the first 6 months because people just can't meet their needs. Those that do manage to live are usually eventually eaten when kept with a clown knife fish due to the drastic size difference between the two. I would strongly recommend you not try to buy another to replace your loss.
Secondly, look at the max length of your remaining two species. Clown knife fish tops out anywhere between 10-20" in the aquarium, though they get even larger in the wild. Depending on the bichir species, you're looking at anywhere between 12" to 39" for an adult fish. Both are predators, which means they are messy eaters, and therefore need more water with frequent changes to do well. Depending on the size of the bichir species and the final adult size of the clown, one very well might even eat the other. Minimum tank size for either fish is about 200 gallons, ideally closer to 300 for the clown. 55 gallons is barely adequate for a juvenile of either species, let alone both in the same tank.
Please, save yourself a huge headache and dead fish later.... Research what it takes to keep both of your remaining fish happy, and get to work ASAP on putting together a tank that will meet those needs. If they need more than you're willing to provide, rehome them or return them to the store. If you do choose to keep them, you'll need a diligent water change schedule and watch those water parameters like a hawk until your new tank is set up and cycled for them.
