On a routine drugs test, Roxicet and heroin will both show as opiates. I'm referring to the standard employer test kit which is widely used, and is also made by the same company who supply the test that police use.
However, as Roxi's are opioids and heroin is an opiate, you are correct that they will show differently on a more elaborate test.
To be honest, there are lots of rumours out there that drinking water and cranberry juice help, but I wouldn't like to guarantee that.
You don't say when your drug test is, do you have time to stay clean for a while before you have to take it? If not, all you can do is try drinking cranberry juice and then lots of water. Go to the bathroom as much as you can, and that may help but, as I said, it's a bit of a long shot.
As for shooting water, please don't ever do that, just drink it normally, and it will be as effective as you can hope for.
Now I don't know you at all, and it's not my place to lecture, you but I don't see this ending in a good way at all. These drugs are dangerous enough alone but together they are deadly. Please get some help before they destroy your life, or worse still...end it.