I emailed my query to seachem support and here's their reply:
Thank you for your email. Yes, if the product is still within the date of expiration, it should still be safe to use! The smell can become a bit stronger over time, but the way the product functions should not be altered. |
Different plants prefer nitrogen in different forms which is why we provide three different forms of nitrate in synthesis. The three different forms help ensure that most plants will have nitrogen available in their preferred form. Typically an imbalance of nutrients will be the culprit of heightened algae growth. If your nitrates are consistently between between 10 and 20 mg/L then dosing synthesis may not be necessary, and holding off on synthesis may help maintain the preferred 10:1 nitrate to phosphate ratio preferred by plants!
I hope this helps!
Seachem Support 10286