I am also a newb in owning freshwater rays. Just wanna share my view on this. I think it does not matter with the size of the display tank. It needs to be large enough for the ray. I think ideally a ray must have at least 1 meter cubic of water and I know this is sometime hard to fulfil because technically if you want to put 2 rays in a tank you must have a tank of 2 meter long and 1 meter wide and 1 meter high. Thats 200cm x100 x 100, equals to 2000 liter of water or 400 gallon. And that's huge. That's why from what I read from this website or from other websites, having rays in a big pond is more ideal. I know not everyone can build a 3x4 meter pond due to size.
But as long as the size of the tank is sufficient I think there wouldnt be a problem.
YES and NO- a 400 gallon is huge for a school of tetras,but probably O.K for one or two rays. Many of these fish are capable of growing a 2 foot disk size which can be tight in a three foot wide tank- especially if they have a roommate.
As for filtration. My experience told me that you must at least have flow rate of 5 times the tank volume. So if you have 400 gallon of tank, you must at least have filter pump that can do 2000 gallon per hour. Having 2 or more filters are better. IMHO, no such thing as over filtration. Well you pay more electricity cost...but that's your financial risk of owning such a big tank.
Nice rule of thumb but there are many different ways to filter a tank and i.e. the purpose of this thread.
For a fish tank water change should be at least once a week 30% i think that's just a general rule of thumb. I own and breed plecos. I do water change every 2 days to try to immitate (not even close maybe) the condition of fast water in rio xingu. Rays come from the same place as plecos (amazon, rio xingu etc) so I think the water change should be around the same interval (twice a week) However I am still a newb in this matter maybe the more experience ray keeprs can tell more.
I agree- the more water changes the better. BUT- there have been successful systems that do very little water changes- I think firemedic has one.
IMHO, I dont think you need RO system for keeping rays. It will become problems more than beneficial. As RO water is just a plain water with minimum or even no mineral contents. You need to put some minerals to make RO water usable for fish. As far as I know people use RO water to soften up the water. As I said above, I used to breed pleco (L333) and I don't use RO water to soften the water as they breed in normal conditioned tap water. So I don't think it's necessary. There are many ray owners in my area that don't use RO system. I think it's expensive and totally un necessary. I don't know if others think differently. I am just speaking out of my experience.
I use all RO- just because there are seasonal chemicals that get added to tap water and I don't want to risk one of those slipping by and contaminating my system. Otherwise I would use tap just to have some extra minerals.
As long as you have adequate filtration, regular water change, appropriate size of tank for your rays. I think they will be happy...just like any other fish.
Bingo- you got it!
Cheers