Comparison: Best Seaweed And Fish Fertilizer (Updated | Dec 1st)

Fish and seaweed blends provide natural sources of nitrogen, phosphorus, and potassium along with trace minerals and hormones that boost root development and stress tolerance. Unlike synthetic fertilizers, they improve soil microbiology, leading to healthier, more resilient plants over the long term. They also release nutrients slowly, reducing the risk of burn and runoff.

The 18‑oz bottle is ideal for small container gardens or trial use, offering a lower upfront cost but higher price per ounce. The 36‑oz size balances price and quantity, making it a good choice for medium‑sized beds or regular seasonal use. The gallon provides the best cost‑per‑ounce ratio for larger gardens or frequent applications, though it requires more storage space.

Both products deliver a 2‑3‑1 N‑P‑K ratio and contain seaweed and fish hydrolysate, but GS Plant Foods is OMRI‑listed for organic certification and emphasizes a higher concentration of trace minerals. Neptune’s Harvest focuses on ease of mixing and a broad application range, while GS Plant Foods highlights soil microbiology benefits. Gardeners seeking certified organic status may prefer GS, whereas those wanting a straightforward, all‑purpose blend might choose Neptune’s Harvest.

Yes, all listed fish and seaweed fertilizers are safe for indoor use when diluted according to the label—typically one tablespoon per gallon of water for houseplants. They also work well on outdoor vegetables, flowers, shrubs, and lawns, providing a versatile solution for any growing environment. Just be sure to avoid over‑application, which can cause leaf spotting.

Check for certifications such as OMRI‑listed or USDA Organic, which indicate compliance with organic standards. Look for statements about sustainably sourced fish and seaweed, and avoid products that list synthetic additives or fillers. Brands like GS Plant Foods and Bloom City often highlight their sourcing and processing methods on the packaging.

For most vegetables and flowering plants, applying every 2–4 weeks during the active growing season provides consistent nutrient support. Indoor plants typically benefit from a bi‑weekly schedule, while larger outdoor lawns may be fed every 3–4 weeks. Adjust frequency based on plant response and soil test results to avoid over‑fertilization.

Seaweed‑rich formulas add plant hormones like cytokinins and auxins, which promote more vigorous foliage, deeper green color, and increased flower production. Fish‑only fertilizers supply higher nitrogen, supporting rapid leaf growth but may not enhance bloom quality as much. Combining both, as in the 2‑3‑1 blends, offers a balanced effect for overall plant health and aesthetics.