Your pH is not good for Urea. It is good for those heavy metals as they will stay in the water and not your creatures. Higher pH with heavy metals like Copper, Zinc, Lead is safer than acid pH which will ionize part of them and make them soluble in water. Plants will uptake those up until some point but they will be affected also. Long before your shrimp have died of Copper poisoning.
I did put Urea in my planted tank. But it is something like 0,5g in a HEAVILY planted 300L tank. No side effects.
I bought mine from the pharmacy as a white salt. Looks like Magnesium Sulfate (aka Epsom salt).