I have a 75g discus and cardinal tank, back in the day bought 3 otto's and a siamese algae eater + nirite snails. I found that all the algae eaters were a bit of over kill, I have since transfered all my cleanup crew to other tanks. Still 1 or 2 ottos hiding in the amazon leaves somewhere. I wouldn't get 6 unless you really like the way they look in the tank. I was looking into congos a while back, I got turned off because a bunch of them seem to get rather large when adult sized. Cardinals.... I think are amazing looking, but they are so finiky. I tried batches and batches of them, but they were always too weak from the local pet stores. I ended up ordering them off liveaquaria.com, and they finally survived the night. Big money drain on those guys.