, go with the syrup storage containers, if you have a choice. Less chance that something has leached into the plastic. On the other hand, I doubt enough residual acid could be still in the material to adversely affect your water parameters. Quite a few people use vinegar to lower PH, but the result is usually temporary.
3g betta, 29g molly, 90g Amer cichlid, 90g community, 137g African cichlid, 220g water garden, 6,500g koi pond