above: a 2Hr Tank in Green Effect, a freshwater tank retailer in Singapore. More difficult species such as the red Eriocaulon quinquangulare do better above 100 umols of PAR on the substrate.
Light strength matters.
Too low and plants are unable to thrive. Too high and the tank becomes far more vulnerable to algae. The relevant measure of strength for a planted tank is the amount of Photosynthetically Active Radiation (PAR) as used in horticulture, but this data is not often found on household bulbs (which generally show the Wattage (a measure of power consumption, not output) and/or Lumens ( a proxy for ‘brightness’ ).
The tricky part is that there is little correlation between PAR, Watts and Lumens...especially when it comes to LEDs bulbs (where there is huge variability in efficiency and efficacy). This leads the following:
A seemingly bright bulb can have very low PAR.
A 24W LED that is stated to be ‘equivalent’ to a 75W incandescent feels strong enough but is often not.
When judged on visual ‘brightness’, we tend to severely under-provide light.
Many ‘plant bulbs’ are no better than ordinary household bulbs.
What strength do we need?
A simple tank with shade tolerant plants (Crypts, Anubias & Java Fern etc.) and a lively fish population can do well with 30 umols of PAR. Lower PAR helps to keep algae at bay in tanks with more fish.
A planted tank with coloured plants would require at least 50-70 umols to bring out richer tones.
A tank with carpets would need around at least 50 umols for healthy growth (with CO2 injection if you want a lush carpet); 80+ would give a very fast growing carpet. Many colored plants get more intense red tones only above 80 umols of PAR or so.
Are household bulbs strong enough?
We took a sample of 5 everyday household bulbs, with the question: given how bright these new LEDs appear, are they strong enough for a planted tank?
The conclusion is that the vast majority of household bulbs would look very bright to the human eye, but would be only suitable for fish-only tanks, or planted tanks with only shade-tolerant plants. To unlock richer hues, it would be far better to go for specialised aquarium lights. Read our review of the latest LEDs here.
How to get PAR data
Unfortunately there is no easy way. You cannot mathematically convert Watts and/or Lumens to PAR. The detailed specifications of household bulbs may contain Information about the bulb’s PAR. Professional aquarium lights generally publish PAR data.
Visual brightness is a very poor proxy for light strength.
A simple tank with shade plants can do well with household bulbs ( you can under-provide by a wide margin and be ok), but is probably insufficient if you plan to grow coloured plants or have invested in CO2 injection to grow more demanding plants.