Sting Ray tanks requirements

I guess this has been asked a few times but wanted to check a couple things:
I have wanted a sting ray for a while, either polka dot, motoro or tea cup and am happy to take things slow....

i have a 5x2x2 in the garage in storage but the kitchen needs refitting before this is setup (or a bigger one again) but I have been allowed a tank under the stairs! I had considered expanding my discus collection but then I wondered if a couple of Rays could fit in there... The maximum size is a 4x2x2ft well actually the depth could be 2.5 or 3 (though this wouldn't be as good viewing). The system would have a ro based continuous water change (well 12 hours a day on a 100gpd to filter)

So I am not going to say is the 4ft enough for 6 months or a year as I think the kitchen will be closer 2 before its done... But would a couple rays be happy in a 4x2x2? Would the additional depth help the Rays at all considering that they will be on continual water change with ro water?

In essence is it a no go for now? would it be possible with only one ray and of the 3 fish above can they be ranked from best suited to least suited?

thankyou

piscavore pisces said:

I wouldn't recommend using ro water. It removes some trace elements beneficial to freshwater fish. Salt water guys uses it because the trace elements are reintroduced with the salt mixes they use.

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Would that include - 79/30 or 50/50 blend? My water quality isn't the best here... Are there any comments on the tank size?

*70/30

can anyone offer any advise, I assumed it would be a no go but from the silence I am now thinking maybe it's not, if a few people could help it would be appreciated,

thanks