How can I tell if the water in my water cubes is good?

Hi everyone!  Good day and happy spring.

I have two 275 gallon food grade totes to capture rain water in.  I cleaned them well when I got them, wrapped them in black plastic so no light can get through, and keep them in a shaded location.  I have a simple filtering system for the water going into the tanks that keep out the debris.

A few months ago, rain has filled the both tanks, but I haven't used the water at all since that time.  The water has been sitting in these tanks undisturbed (no outflow, no inflow, no treatment of any kind) and now that I need it, I want to use it.  However, I want to make sure the water is good before I use it only, I do not know how.  

I'm asking because I noticed some slimy stuff growing on the outside of the tank where I have a fitting that goes between the outflow of the tank and a spigot.  And too, because I never treated the water going into the tanks straight from the gutters.  so it's just rain water sitting in an IBC tote.  The slimy stuff is definitely on the outside of the tank.  The water looks find - it's not discolored or anything and it doesn't have a smell to it.  But I know too that you can't just look and know if it's ok.

How can I test the water to make sure that 1) I won't harm or kill my plants should it not be right for some reason, and 2) I won't make myself or anyone who eats the food from the garden sick, should something be living in the water that is harmful to humans.

I know I can go through the county's water testing program (I will do this), but my question is more related to, will it harm my plants or cause harm if my plants are ingested by a human?  I'm afraid the county testing program will only tell me if it's drinkable or not.  What I need to know is, can I safely use it to water my plants and then safely eat my plants.

Thanks everyone!
Happy growing.  :)

The water is almost certainly safe if you don't drink it straight from the tank.  It would need to be filtered or boiled for human consumption but is safe to use to water plants.  The slime is just algae which will grow wherever there is water and sunlight.

[quote=Logan Therrion]  What I need to know is, can I safely use it to water my plants and then safely eat my plants.
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I suspect that all plastics have the capability to leach minute amounts of chemicals into water under the right conditions, but by protecting the tank from sun and heat, I'd guess that there would be no problem using the water in your garden. Around here, people use those tanks for water transport and storage all the time. I haven't heard anyone say that the water smelled "funny".

Id use the water.

Logan Therrion wrote:  What I need to know is, can I safely use it to water my plants and then safely eat my plants.

Yes, it will be fine.  The slime you see is either algae or bacteria, neither of which will be harmful to your plants.  

People these days get overly concerned with bacteria.  Bacteria is everywhere, in the soil, in the rain, on your skin, inside your intestines (it NEEDS to be there, otherwise you can't digest most foods).

Trying to avoid bacteria at all costs is actually bad for your health.