April 2026 | Reading time: approx. 8 minutes
What Is Fish Sh!t and Why Does Every Serious Grower Use It?
There’s a product sitting on the shelf of virtually every professional indoor grow room in the country, and its name is exactly what it sounds like. Fish Sh!t Organic Soil Conditioner, made by Fish Head Farms, has built a devoted following among soil and hydroponic growers alike — not through clever marketing, but through results that are genuinely hard to argue with.
At its core, Fish Sh!t is a living microbial inoculant. It’s not a fertiliser. It contains no NPK. What it does contain is over 4,000 distinct species of beneficial microbes — bacteria, fungi, and protozoa — derived from the waste of tilapia fish raised in a proprietary closed-loop aquaponic system on a specialised organic diet. The resulting liquid is essentially a concentrated ecosystem in a bottle, one that transforms the biology of your root zone when applied regularly.
The reason growers keep coming back to it is simple: once you understand how plant nutrition actually works at the microbial level, you realise that feeding the soil biology is just as important as feeding the plant directly. Fish Sh!t does exactly that.
The Science: What Fish Sh!t Actually Does in the Root Zone
To understand why Fish Sh!t works, you need to understand the rhizosphere — the thin zone of soil or growing medium that surrounds plant roots. This is one of the most biologically active environments on Earth. In healthy soil, this zone is teeming with billions of microbes that form a complex, mutually beneficial relationship with plant roots.
Research from institutions including Rutgers University has identified a process called the rhizophagy cycle, in which plants actively ‘farm’ soil microbes. Roots secrete sugars and organic compounds to attract microbes, then absorb and strip nutrients from them before ejecting them back into the soil to gather more. It’s an extraordinarily efficient nutrient delivery system — and Fish Sh!t turbocharges it by dramatically increasing the microbial population available to the plant.
In practical terms, the microbes in Fish Sh!t do three critical things:
- Fix atmospheric nitrogen: Certain bacterial species in Fish Sh!t convert nitrogen from the air into a form plants can absorb directly, supplementing your nutrient programme.
- Solubilise locked phosphorus: Phosphorus frequently becomes chemically bound in growing media, particularly in older soil or after heavy feeding. Specific microbes in Fish Sh!t break these bonds, unlocking nutrition that would otherwise be wasted.
- Boost terpene and essential oil production: Mycorrhizal and bacterial activity in the root zone has been linked to increased production of terpenes, flavonoids, and aromatic compounds by up to 20% — critical for flower quality.
Fish Sh!t vs Other Organic Soil Conditioners
The microbial inoculant category has grown significantly, and there are now several products competing for the same shelf space. The key differentiators for Fish Sh!t are its microbial diversity (4,000+ species versus single-digit strains in many competitors), its fish manure origin — which produces a distinctly different microbial community to plant-based ferments like EM-1 — and its OMRI Listed certification confirming it meets the standards for certified organic growing.
Critically, Fish Sh!t can be used at every single watering from seed to flush. Most powder-based products like Great White are only applied at transplant. Voodoo Juice from Advanced Nutrients contains just 5 bacterial strains and isn’t OMRI certified. EM-1 (Effective Microorganisms) is a legitimate alternative but uses a plant-based ferment rather than fish manure, producing a different microbial profile. Fish Sh!t is also fully compatible with synthetic nutrients — something that matters to a lot of growers running high-performance mineral programmes.
Complete Fish Sh!t Dosing Guide
The most common mistake with Fish Sh!t is inconsistent application. Because this is a living product, it builds up in the medium over time — the microbial colonies establish and grow with repeated inoculation. One or two applications won’t give you the full benefit. Use it at every watering.
| Growth Stage | Dose | How to Apply | Media |
|---|---|---|---|
| Seed Germination | 1 ml per gallon (3.8L) | Add to soak water or germination tray | All media |
| Clones / Cuttings | 2 ml per gallon | Prepare 24 hours before planting; soak rockwool or plug | All media |
| Root Dip (intensive) | 50 ml per 50 ml water (1:1) | Quick 30-second dip before transplanting | All media |
| Transplant Aid | 6 ml per gallon | Water at the base of the plant to reduce shock | All media |
| Vegetative Growth | 3–6 ml per gallon | Use every watering throughout veg | All media |
| Bloom / Flower | 2–4 ml per gallon | Continue every watering; supports terpene and resin production | All media |
| Final Flush | 3–6 ml per gallon | Helps break down residual salt build-up in medium | Soil / Coco |
| Hydroponic Reservoir | 2–4 ml per gallon | Add consistently to reservoir; re-inoculate after each change | Hydro / DWC |
Three rules before you apply: (1) Shake the bottle vigorously every time — the microbes settle between uses. (2) pH your water before adding Fish Sh!t — a stable pH environment helps the microbes establish. (3) Do not use as a foliar spray — this product works in the root zone, not on leaves.
How to Use Fish Sh!t: Application by Growing System
Soil and Organic Grows
Fish Sh!t is in its natural home in living soil. The microbial diversity it introduces integrates beautifully with the existing biology in quality soil mixes such as Biobizz All-Mix or similar pre-amended substrates. Apply at every watering at 3–6 ml per gallon during veg, reducing to 2–4 ml during bloom. The microbes will colonise and continue to multiply between applications, building an increasingly healthy rhizosphere as the grow progresses.
One particular benefit in soil grows is the flush stage. A final flush with Fish Sh!t at 3–6 ml per gallon actively assists the breakdown of residual nutrient salts in the medium, which can otherwise affect final flavour. This is something most other microbial products don’t specifically target.
Coco Coir
Coco is a sterile medium — it has no native biology. This is where a microbial inoculant like Fish Sh!t makes the most dramatic difference. Without it, you’re entirely dependent on your nutrient line for plant health. With regular Fish Sh!t applications from the first watering, you begin to establish a living rhizosphere that works alongside your nutrients rather than in isolation from them. Use at 3–6 ml per gallon every watering, adjusted to 2–4 ml in bloom.
Hydroponics (DWC, NFT, Recirculating)
Fish Sh!t is specifically formulated to be hydro-compatible. Use 2–4 ml per gallon consistently in your reservoir. The microbes it introduces are resilient enough to survive in solution and colonise the root zone even in recirculating systems. Re-inoculate with a fresh dose after each reservoir change. After a water-only flush, add a fresh dose to re-establish the colony.
Compatibility: What Can You Use Fish Sh!t With?
Fish Sh!t is 100% compatible with all nutrient lines, whether organic or synthetic. It doesn’t compete with your existing programme — in fact, it can actively help synthetic growers by breaking down salt accumulation that builds up with heavy mineral feeding. It works well alongside mycorrhizal products like Great White or Myco Chum for a broader spectrum of root zone biology. Always pH your water before adding Fish Sh!t rather than after.
Watch: Testing Fish Sh!t in the Garden
For a practical demonstration of the product in action and a look at the results growers are achieving, this independent test is one of the most thorough real-world comparisons available:
Common Questions About Fish Sh!t
Does Fish Sh!t smell bad?
Yes — and that’s completely normal. Fish Sh!t is a fermented, living product, and different batches can have quite different aromas ranging from mild and earthy to fairly pungent. The smell doesn’t indicate anything about potency or quality. The microbial count is verified through rigorous testing before bottling regardless of the aroma.
Can I use Fish Sh!t during the flush?
Yes, and we’d actively recommend it. A flush with Fish Sh!t at 3–6 ml per gallon helps the microbial community break down residual nutrient salts in the medium rather than simply washing them out. Many growers report this produces a cleaner-tasting final product compared to water-only flushing.
How long does a bottle last?
At standard dosing of 3–6 ml per gallon, a 1 litre bottle will cover approximately 45–90 gallons of water. For most home growers watering 4–8 plants with 2–5 gallons per session, a litre bottle will typically last 4–8 weeks.
Is Fish Sh!t suitable for beginners?
It’s one of the most beginner-friendly additives available precisely because you can’t really overuse it. Unlike synthetic additives where overdosing can cause nutrient burn, Fish Sh!t is a living product — excess microbes simply don’t establish if the environment can’t support them. The worst case is wasted product. The upside is that even at lower doses than recommended, you’ll see improvement in root health and plant vigour within a couple of weeks.
Is it OMRI certified?
Yes. Fish Sh!t is OMRI Listed, confirming it meets the standards for certified organic production. It also holds USDA BioPreferred designation, which confirms it’s derived from renewable biological sources — not a synthetic formulation with organic marketing language.
Shop Fish Sh!t Organic Soil Conditioner
We stock the full range of Fish Sh!t at The Horticulture Company, including the 250ml starter size for growers trying it for the first time and larger volumes for commercial or multi-run use. Free delivery on orders over £200.
Browse Fish Sh!t Organic Soil Conditioner at The Horticulture Company →