[ray-the-pilot]

I would be a little cautious about using those old chemicals. While the chemicals themselves are quite stable the packaging probably is not.

Before you use them make sure that you do not see any corrosion of the container or closure. Check the bottle liner. This is usually the weakest part of the packaging. If it looks corroded I wouldn’t use it.

One other thing to think about. If you are using technical grade chemicals, there is a lot of other stuff in the bottle that can be degrading over time. You have no way of knowing what is going on with those things.

 

[ray-the-pilot]

Does anyone obey a "use before" date on a container of table salt? I doubt there even is a "use before" date on them. Our basic NPK fertilizers are just salts, so they can be treated just as we treat table salt. Trace element mixes are different because they are almost always chelated mixes, and the chelator is an organic chemical, not a salt.

I agree but I wouldn't use a 2 year old shaker full of technical grade salt with a rusty cap on my steak. Maybe it is just me but I don't mind paying $10 for peace of mind.

 

[ray-the-pilot]

The ferts were stored in large plastic zip-loc bags in the dark. Would the plastic bags do any harm to the chemicals?

I can't say if your ferts are good or not. I doubt that anyone can say for sure.
But at least you should look at the packages. Are they intact with out any evidence of damage or reactivity? Is the product still physically OK or is it a solid block? Smell the KNO3 do you detect any ammonia? When you dissolve the ferts in water are the solutions clear or do you see some precipitate?

Despite what anyone says, even inorganic salts do not last forever or even hundreds of years. They can react in lots of ways that make them ineffective or possibly bad for your fish and plants. This is especially true if you are using less pure chemicals with lots of unknown impurities.

This is my suggestion. You can buy 1lb. of K2SO4, KH2PO4 and KNO3 for $10. That seems like a pretty low price to pay for peace of mind especially if you are planning to spend big buck on fish.

Then take the 2 year old chemicals and sell them 50% off to anyone who thinks they are still good and let them use them.