Tats are where it's at in my opinion. Armatus are great but Tats shoal very well. You can mix other species with Tats, Armatus you aren't going to be as lucky.
Get a shoal of Tats and a shoal of Rhaphiodon. You can't go wrong with this combo.
SharkAquarium had a tank full of Tats last month.The Armatus is the best looking IMHO but they are antisocial and mine does not take water changes very well,it takes about two days for him settle down and recover,so they are not the most hardy.The other two accept water changes normally.
Ive owned all three species of Hydrolycus and the armatus is the most hardy by far. I also like them the most, an awesome fish! If you must co-hab best to keep tats.
Mine is just fine no matter what. Ive kept him with rays that got large water changes often and Ive kept him with other fish that received very little and very few water changes. Ive owned two different armatus and never noticed any problems with water changes.
I have only owned tat's and scomb's. And i have killed about a dozen scombs over the years with little to no explanation. I have only had 2 scombs that i did not kill and grew very well for a couple years. And then I have and still do have 3 tat's with no problems. So i would have to say Tat's are hardier to me.
Its kinda hard for me to decide which of the two is my favorite at the momment,tats and mats both have their own kind of beauty but I think tats show better in groups and the armatus does it on its own.