What's the deal with RO/DI water?

I make my water in either a 5 gallon bucket (with my old 20) And now with a 20 gallon brute, i just dump it all in there, and hold a powerhead in there, moving it by hand to get the clumps at the bottom.

 

What you're describing sounds like you're seeing precipitation of your salt mix. Unless it's simply a bad batch, this is an artifact of how you're mixing it. It is NOT due to your water.

It sounds like you are mixing properly, (adding slowly, continuously running a pump), so I don't quite know what to suggest. Heating to temp prior to mixing may certainly help.

Other thoughts that come to mind:
You say you're mixing a cup at a time, but are you just dumping that in or slowly pouring it in? Precipitation happens due to exceeding the saturation capacity of the water. If you're just dumping it in, it could be locally precipitating where it's super-saturated. NEVER add it all at once, or add water to your salt.
How are you checking salinity? (with what type of instrument)? Are you checking as you mix, narrowing in your target? It might be possible you're way exceeding your target, which might not be a fault of your own if you're using a sub-par instrument.

 

Sounds like you are doing things right. I slowly pour my salt in a cup at a time and let it dissolve well before adding the next and have no problems with my Oceanic salt. It is usually clear enough to use withing the hour, I never store it or let it sit overnight.
When I used IO I did sometimes get cloudiness but not normally.

 

I use a refractometer to check which is calibrated to the ro water.

 

Thanks for all the advice guys. From now on I will make sure I shake up the salt mix before using, heat water to temp and then continue to add the salt mix slowly to the water making sure each cup desolves first before adding another. Hopefully the combination of these steps will alleviate the problem.