Minimum photoperiod for first few weeks

[Ajm200]

Quick question. What is the minimum photoperiod I can get away with for a planted tank?

Currently have 108w t5 running for 8 hrs over a 2 week old 500l tank. This is already down a bit from where I started.
Noticed a few diatoms on the spraybars today (plants are ok so far touch wood) and want to reduce the photoperiod to 5 or 6hrs. Will this be ok or will I risk damaging the plants?

In the past I've had tanks covered in diatoms at the two week stage so think this one is going reasonably well.

I'd really appreciate your advice

thanks
Amanda

 

[ceg4048]

Amanda,
It's impossible to calculate what the sufficient photoperiod is. To survive, each plant must produce at least as much sugar as it burns for daily living. To grow it must produce more sugar than it burns. To grow fast it must produce a lot more sugar than it burns. Since we don't have the tools to measure either the sugar consumption or the sugar production, it would be total fantasy to say X watts of T5 for Y hours is good. Even if we guessed right for a given species, or for an individual specimen in the tank, those numbers wouldn't necessarily apply to all species or for all specimens in that tank (or for any specimen or species in any other tank).

Try 6 hours and see what happens. You should see a decline in growth rate unilaterally but some plants may not produce enough food. Just monitor. Since photosynthesis (and therefore food production) depends on both light and CO2 it's often difficult to tell the difference between not enough light and not enough CO2. If it's not enough light you'll observe that plant mass simply disappears, i.e. stems become thin, leaves don't grow and so forth. If it's not enough CO2 then you'll see distress such as browning, mushiness etc.

You would do well to think about light as a hail storm. High wattage means fast, large hail balls (maybe golf ball or even cricket ball size) and low wattage means pea size. Think about a bucket placed out in the open. How long does it take to fill the bucket with ice? Large fast hail balls fill it quickly while small pea size balls take longer. If I knew exactly how heavy the hail stones were, how fast they were traveling then I could calculate how long it would take to fill the bucket to a certain weight of ice. Without knowing these numbers I'd just have to put the bucket out there and see....

Cheers,

 

[Ajm200]

Thanks for the reply. I'll reduce the light to 7 hours and if there are no ill effects and the diatoms continue I'll reduce it again. Everything's growing fast at the moment so I guess it will be ok.

Thanks for you help. Now I have a better idea of what to look for.