Stingrays!

cant keep them with oscars

It really depends on the oscar and the ray. But since it is only a 135, already has an oscar in it, and rays need VERY high water quality it would probably not work well in that setup at all.

also oscars become aggressive as they get older. i spoke to a few ray breeders and they all told me no aggressive fish can be kept with them, especially cichlids. my friend lost 2 leopoldi due to a curious silver arowana...who would have guessed. you have to be careful when dealing with these sensitive fish.

You might enjoy looking at this site. Doesn't answer your question, but it sure is cool.
http://www.aquarticles.com/articles/people/Wyne_Frank_Nico.html

I have had a couple different retic rays before. I will say that a bare bottom tank is the easiest way to keep them, but a sand bottom looks the best and they the rays like it the best. I personally would not get another retic ray because all the ones I have had were picky eaters. Motoro rays are a little more, but they will eat different stuff and are more owner friendly. Keep the water in the low 80s and make sure you use a heater cover. Cover any pumps inlets as I had one ray get stuck to the end of a powerhead. He healed, but I thought I was going to lose him. Make sure you have live blackworms or live earthworms when you get your ray as this is probably the only thing you will get them to eat at first. If you can get captive bred rays, they will take food better and you should not have to worry about disease when you get them. If you have any specific question you can pm me.

Actually I think any further information should be discussed in threads so everyone can benefit.

One problem with motoros is their max size is much larger than P. reticulatus.

I think bowl-feeding is one of the best things you can do for picky eaters. Once they begin to associate the bowl with food it makes it much easier to switch that food.