Seachem Prime Complete Guide: Dosing and Myths

Seachem Prime sits in almost every serious Singapore aquarist’s cabinet, yet half of what hobbyists believe about it is incorrect. This Seachem Prime complete guide from Gensou Aquascaping at 5 Everton Park covers what Prime actually does, how to dose it precisely for SG chloramine-treated PUB water, and the myths that circulate on forums — including the infamous “Prime kills your biofilter” claim. After two decades of using Prime across hundreds of client tanks, the patterns are clear enough to write plainly.

What Prime Actually Does

Prime is a polyamine-based reducing agent. Four documented functions: neutralises chlorine via reduction, breaks chloramine bonds and binds the released ammonia, converts nitrite and nitrate to temporarily non-toxic forms (within limits), and chelates heavy metals like copper and zinc. The ammonia binding is time-limited — roughly 24-48 hours before the bound form releases back to the free state, giving biofilter bacteria a grace window to process the load.

Standard Dose

One millilitre per 40 litres (2 drops per gallon US). On a 100 L tank at 50 per cent water change, dose 1.25 ml — roughly a quarter of the 5 ml cap. For SG tap water testing at 0.5-1.0 ppm total chlorine/chloramine, the standard dose has roughly 2-3x safety margin. Dosing before refilling, directly into the tank is the current Seachem recommendation for Python-fill setups; dose into the bucket for bucket changes.

Emergency Dose

Up to 5x standard dose is safe during ammonia or nitrite crises. That translates to 5 ml per 40 litres, or a full cap for 200 litres. Prime binds up to 1 ppm total ammonia/nitrite at standard dose, 5 ppm at emergency dose. During a fishless cycling crisis or stocking accident, 5x Prime buys 48 hours for emergency water changes or biofilter rescue. Never exceed 5x — oxygen-binding side effects increase beyond that ratio.

Myth — Prime Kills Biofilter

False. Prime binds ammonia into a form the biofilter can still process; nitrifying bacteria consume the bound ammonia over 24-48 hours. Lab studies (and two decades of field use) show no biofilter crash from standard Prime dosing. The myth may stem from confusion with chlorine/chloramine itself killing biofilter bacteria — Prime prevents that, it does not cause it.

Myth — Prime Creates False Ammonia Readings

Partially true but misunderstood. API ammonia tests use Nessler or salicylate reagents that detect total ammonia (bound plus free). Prime binds ammonia into a form fish gills cannot access, but the test still reads it as present. A reading of 1 ppm ammonia after Prime dosing is not a dosing failure — it is the test seeing what is actually there, just in a non-toxic form. Seachem MultiTest Ammonia ($42) distinguishes free from total for accurate emergency diagnostics.

Myth — Prime Works Forever

False. Bound ammonia releases back to free form after 24-48 hours. During an active ammonia spike, Prime must be redosed every 24 hours until the biofilter catches up or water changes resolve the source. Dosing Prime once and assuming a 1 ppm tank will remain safe for a week leads to fish loss on day 3-4.

Shelf Life and Activity

Prime maintains full activity for 5 years unopened, 2-3 years after opening if stored under 30°C and away from direct light. SG ambient conditions stress Prime harder than temperate climates — a bottle left on a tank cabinet in direct afternoon sun may degrade in 12 months. Store in a dark cupboard. The sulfur smell is normal — its absence signals degradation.

Dosing for Chloramine Specifically

PUB chloramine averages 0.5-1.0 ppm total chlorine. Standard Prime dose neutralises up to 2 ppm, giving comfortable margin. For new-mains areas post-PUB flushing or annual system maintenance, chlorine can temporarily spike to 2-3 ppm — dose 2x Prime after notification emails. For first-fill of a new tank with fresh PUB water, dose 1.5x standard to be conservative.

Prime vs Safe Economics

Prime 500 ml $45 treats 20,000 litres. Safe 250 g $95 treats 500,000 litres. For a 150 L tank with 50 per cent weekly changes, Prime 500 ml lasts 5 years — approaching its shelf life regardless. Buying Safe for this scale wastes 95 per cent of the product. Prime is the correct size choice for most SG tanks under 400 L. Fish rooms and ponds justify Safe.

When Not to Use Prime

Reef systems with copper-based medications — Prime chelates copper, neutralising therapeutic doses. Dose Prime only during pre-treatment water changes, not during active copper medication runs. Systems with chemical dosing regimens that require un-chelated trace metals also need caution. For general freshwater, planted and soft-coral reef tanks, Prime is safe in every standard dosing scenario.

Buying Authentic in SG

Counterfeit Prime exists on Shopee — counterfeit bottles typically have incorrect cap colours, blurred printing and watery consistency. Genuine Prime is thick, syrupy and golden-brown. Buy from C328 Clementi ($45 for 500 ml), Green Chapter Jurong West, or authorised Shopee sellers with Seachem authentication. Avoid listings pricing below $35 for 500 ml — that is typically diluted or expired stock.

Singapore Sourcing Summary

Seachem Prime 100 ml $18, 500 ml $45, 2 L $85, 4 L $155 at C328 Clementi and authorised Shopee sellers. Seachem Safe 50 g $32, 250 g $95, 1 kg $280 specialist importers. Seachem MultiTest Ammonia $42 for free/total ammonia distinction. For most SG hobbyists running 1-2 tanks totalling under 400 L, Prime 500 ml is the correct single purchase — it covers standard water changes for 2-3 years, handles emergencies at 5x dose, and stores well if kept cool and dark.

Related Reading

• Water Conditioner Fish Tank Guide
• Fish Tank Dechlorinator Guide
• Aquarium Ammonia Management
• Fishless Cycling Guide Singapore
• Fish Tank Water Change Guide Singapore