first thing to do is doing the right thing, just remember never trust the water from your local water supply, no matter how clean it is, it always contains chemical which could harms your fish, specially chlorine, its the worst poison for your fish.
Similar thing happen to me before, and thats when I had so little knowledge about keeping fish in the tank. I lost 3 ornates and other bichirs from chlorine. When it happened I couldnt save them, but I was able to save the rest of my bichirs.
For immediate treatment I quickly replace the water in my tank with water from my fish pond in the backyard, I know that the water is better compare to tap water, then I put as many oxygen as possible in the tank, slowly as my bichirs improving, I added salt in the water, a teaspoon per gallon, and always dissolve the salt first before you mix them into the water. I would say the number of salt given depending on how bad is the condition of your OB. Dont over due it, again just remember OXYGEN level in the water is very very important, eversince your bichirs breathing fast.
If you dont have any clean water then buy bottle water as many as you need. Its better to separate your OB into a separate tank and treat it intensively.
In a separate tank, I would lower the water level, cause bichirs always come to surface to breath, so I guess its better to shorter the distance between bottom of tank and the surface.
This is all I could say at this moment, dont bother feeding your OB at this moment since its conditions are not well, dont worry it wont kill your OB for no eating in awhile. If you put to much food in the water and our OB didnt eat them the it will just make your water even worse.
I use similar submersible pump in my tank, In fact I have few of those running.
Hope you find this information useful and your OB getting well. I use this methods so far when the worse thing happen. If everything goes well, you may wanna rethink about the set up in your tank, including filters and number of water chances. Find put if anything odd, and have a look at people tread in this forum, thats how I learn till now.
Keep us updates.
cheers