Use the HOB filters for now but if I were you get an fx5. I have had stingrays in my 135g, right now I only have 1 male marble ray but in that tank I'm running two FX5's. That's 800g of filtration on a 135g, 6ft tank. Overkill, yes....but my water is always clean and I never have problems with my ph,gh etc...etc....etc....
Maybe I'm a bit late but I was reading through the threads that you have in the piranha section, and I just wanted to throw my .02 in. If you can't properly cycle a 30 gallon for two small natt's what in the world made you go for a 125 with a motoro. You realize that you went from fish that are pretty much bullet proof to a fish that most beginners cannot properly house. Water conditions is the key with rays, and your choice of filter is not generally what I would consider efficeint. I also think that the blue sand makes the whole setup appear extremely cheesy.
Honestly, its your second tank, with such little filtration, I suggest you rethink your plans. Little filtration with 10% weekly changes (mentioned in your piranha thread as your WC schedule) is just asking for a disaster.
My motoro came in and its amazing!!! I have tested the water everyday and the parameters are good but i am keeping a close eye on them.
She is already eating bloodworms and will come up to the surface when you put them in the water. She also is eating off of tongs.
I am adding an in-tank filter and was wondering if i should go with a Fluval plus 2. Its rated for around 35 gallons and i am already running 2X emperor 400 marinlands running at 400 gallons an hour. Would this be better? Thanks