API pH Down
Pack size
118 mL
Cost per litre
Under 1c
Water type
Freshwater
Pack size
473 mL
Cost per litre
Under 1c
Water type
Freshwater
API pH Down
A concentrated acid dose that moves pH down in millilitres, not capfuls. API pH Down is a sulfuric-acid-based liquid (under 10% by weight) that lowers freshwater aquarium pH for soft-water species such as tetras, discus and Amazonian community fish. It doses at 1 mL per 10 US gallons (37.85 L), sold here in 118 mL and 473 mL Professional Size bottles.
Key features
- Small doses, real effect – 1 mL per 10 US gallons (37.85 L) is the full working dose; this is a concentrate, not a diluted conditioner.
- Two bottle sizes – 118 mL for routine top-ups, 473 mL Professional Size for larger systems or frequent dosing.
- Under 1c per litre of treated water at API's label dose, on either bottle size.
- Freshwater aquariums only – API does not rate this product for marine or reef systems.
- Corrosive concentrate – this is an acid product (see Safety) and is dosed by millilitres, not poured freely.
Which size do I need?
| Bottle | Doses at label rate | Approx. water treated | Approx. cost per litre treated |
|---|---|---|---|
| 118 mL | ~118 doses of 1 mL | ~4,466 L | Under 1c per litre |
| 473 mL Professional Size | ~473 doses of 1 mL | ~17,900 L | Under 1c per litre |
Both sizes work out to a fraction of a cent per litre of treated water at the label dose; the 473 mL Professional Size is the better buy per litre if you dose often enough to use it before you'd next need it. These figures are calculated from API's own 1 mL per 10 US gallon dose rate and each bottle's stated volume – API does not publish a pack yield figure itself.
How much do I use?
| Aquarium volume | Dose at API's label rate |
|---|---|
| 10 US gal (38 L) | 1 mL |
| 20 US gal (76 L) | 2 mL |
| 55 US gal (208 L) | 5.5 mL |
| 100 US gal (379 L) | 10 mL |
Test pH before adding another dose. API's guidance is not to move pH by more than 0.2 in any 24-hour period – dose, wait, retest, and repeat only if needed rather than chasing a target in one application.
Specifications
| Active ingredient | Sulfuric acid, less than 10% by weight |
|---|---|
| Form | Liquid concentrate |
| Dose rate | 1 mL per 10 US gallons (37.85 L) |
| Water type | Freshwater aquariums |
| Pack sizes sold here | 118 mL, 473 mL Professional Size |
| Maximum recommended pH change | 0.2 per 24 hours |
| Hazard classification | Corrosive – skin/eye damage category 1A, corrosive to metals |
| Freight | Standard |
Suitable for, and not suitable for
Suitable for:
- Freshwater community tanks that need a lower, more acidic pH – tetras, discus, rams and other soft-water species
- Gradual, tested pH correction alongside a KH (carbonate hardness) reading
Not suitable for:
- Marine or reef aquariums – not rated by API for saltwater use
- Hard, high-KH tap water as a permanent fix – the buffering will fight every dose; lowering KH is the underlying correction
- Dosing at the same time as a pH-raising product – API warns this can cause erratic pH swings
Safety
API's Safety Data Sheet classifies this product as corrosive: it may be corrosive to metals and causes severe skin burns and eye damage on contact. Handle it as an acid, not a conditioner:
- Keep in the original container, tightly sealed, out of reach of children
- Wear eye protection and gloves when dosing
- Flush skin or eyes with water immediately on contact and seek medical attention
- Never add water to the concentrate; add the measured dose to tank water, not the reverse
- Do not dose at the same time as a pH-raising product
Goes well with
- API Freshwater Master Multi Test Kit 5 in 1 – tests standard-range pH and KH, so you can see why a tank resists adjustment and confirm each dose has worked.
- API pH Proper 6.5 or API pH Proper 7.0 – a buffered powder that holds pH at a set point once pH Down has brought it into range, instead of one-shot dosing indefinitely.
Warranty and delivery
This is a consumable, so no manufacturer warranty period applies. That does not affect your rights under the Australian Consumer Law: if the product is faulty, unsafe or not as described, you are entitled to a remedy.
Ships as standard freight. As a corrosive liquid, it travels in its original sealed packaging.
Common questions
Can I use API pH Down in a marine or reef tank?
No. API rates this product for freshwater aquariums only. Marine and reef pH should be managed through the salt mix and buffering products designed for those systems.
How much will one dose lower my pH?
API does not publish a fixed pH-per-mL figure because the result depends on your water's KH (carbonate hardness) – harder water resists the change more. Dose at the label rate, wait, and retest rather than assuming a fixed drop.
Why does my pH keep drifting back up after dosing?
High KH acts as a buffer and pulls pH back toward its original point. If you're redosing frequently, testing and reducing KH is the more stable long-term fix rather than repeated acid dosing.
Can I use pH Down and pH Up together?
No. API warns against dosing a pH-raising and pH-lowering product at the same time, as the two can cause erratic pH swings rather than settling at a stable point.
Is API pH Down safe to handle?
It's classified as corrosive on API's Safety Data Sheet – it can cause severe skin and eye burns undiluted. Measure it carefully, avoid skin and eye contact, and always add it to tank water rather than adding water to the concentrate.
