Amazon [amazon.com] has 4-Oz API TAP WATER CONDITIONER Aquarium Water Conditioner for $4.78 - $2.39 when you 'clip' the coupon on product page - 5% when you check out via Subscribe & Save = $2.15.
Shipping is free with Prime or on $35+ orders.

Price
$4.53 lower (68% savings) than the list price of $6.68
$2.63 lower (55% savings) than the previous price of $4.78

Savings
Save an extra $2.39 on your first Subscribe and Save order. (check and activate [amazon.com])

Subscribe & Save
5%: $2.15
10%: $1.91
Add to next delivery [amazon.com] (with free shipping)
fillers [amazon.com]

Customer reviews
4.7⭐ / 1,302
1,000+ bought in past month

amazon.com/dp/B0002ASF50 [amazon.com]

My other deals