i_got_no_idea, just stop.
retics are no harder to keep than the others. they simply have a smaller mouth so their foods have to be served smaller.
the only reason they've been "hard to keep" by anyone is, since they're the cheapest ray to purchase and the importers make nothing off them, they ship them with less care than the other more prized specimens. thus, they usually come in bad health, ammonia burn, DOA, or beat to hell. then some poor sap buys the ray because of an impulse buy and a week later, it dies of the injuries already sustained from transit/exporter storage conditions.
if you know what ray you're buying (see it in the store yourself over the course of months, watch it grow and eat, be active, thrive, then it will generally not end up dead, unless you do something very wrong.
stereotypes are not always true.