Actually the best algae eater for Bichirs are a large enough Pleco, that will actually be happy eating algae…. I wouldn’t typically recommend the common Pleco, as they get too large, but they do continue to eat algae after they get big… I have one that’s about 8 - 10 inches long in with my Bichir, and mine still does a great job keeping the acrylic tank spotless, even in the rounded corners…
I would stay away from the bushy’s… they are also great tank cleaners, but don’t get big enough, and you’ll have problems with your bichirs wanting to put them in their mouths later on…
There are lots of better looking pleco’s that will get big enough, to live with your bichirs, like the royals but most either don’t eat much algae, or eat other things as they get bigger…
Common pleco’s are poop machines, and at 18 inches, you may need a chainsaw, and they will require extra sized filters to support them…
If you currently have 3 bichirs in one tank, and wanted to add a common pleco, you’re going to need larger than a 75 gallon tank at some point, with a lot of filtration… but a common pleco would be a good tank mate…
You’ll need to buy one that is currently as big as your biggest bichir, and as these often outgrow most peoples tanks, you should try to adopt one that’s already big…. They get a bad rap, because they have become an invasive species, because they outgrow people’s tanks, and they dump them in lakes, rivers, or flush them down toilets, so I recommend you study a little, and don’t get one, unless you are as committed to it, and plan for it to live long, and grow huge, as much as you would be your bichirs