Watts Per Gallon?

[JDFish]

I recently bought 2 30w floodlights from amazon. However I am looking into purchasing an AQUAEL 32w led. It has 8000k and just looks overall cleaner. My tank is 160 litres but probably will only have around 110 litres of water in it. The lights are expensive and my seller is telling me I need 2 of them and not 1. IM growing easy plants except for ludwigia arctura and Monte carlo. (I dont really care if the ludwigia dies but I do want a carpet from my monte carlo). Tank is 42.7 US gallons and i remember reading somewhere easy plants need 0.5 watts per gallon of water. Advice is much appreciated.

 

[Colin_T]

Forget about the 0.5 watt per gallon. What are the tank dimensions? How tall is the tank? Tank height is what you need to consider with light. The taller/ higher the tank, the more light you need to penetrate the water.
eg: if the tank is only 12 inches high and only has 8-10inches of water (in height) you will have heaps of light from 30 watts. But if the tank is 2ft high then you need about 150+ watts if you want bright light to get to the plants on the substrate.

Globes with 8000K are not really necessary for freshwater tanks. They have more blue light and are designed for corals in marine tanks. 6500K is fine for freshwater and are usually cheaper than 8000K, and they should both produce a bright white light.

 

[Lunar Jetman]

You need to be careful with Watts per Gallon calculations as they can be a bit over simplistic and not take into account depth of tank, type of lights etc. I think it’s different with LED lighting as well but it’s been a while since I looked into LED lights for my tank.

However, as a general guide, I usually use the following guide for lighting:

Up to 1 watt of light per gallon of tank volume: Low
Up to 2 watts: Medium
Up to 3 watts: bright
4 watts + : Very bright

Have a look at this calculator that should give you a more accurate rating for your tank

https://rotalabutterfly.com/light-calculator.php

 

[JDFish]

My tank is 50cm high the light would be around 55cm. I have 8cm of substrate so in total around 50cm is the led to the substrate. Is this ok

 

[Colin_T]

30 watts is not enough for that height. I would go to a hardware store and look at their 50, 75 & 100watt LED spotlights. You can buy some with cords and switches and you just hang them above the tank. Try to find one with a 6500K rating. Take one home and hang it above your tank and turn it on. Put you hand just under the water surface and see if there is a strong shadow on the substrate. If there is then the light should be fine. If there is no shadow or only a pale shadow, take the light back and swap it for a higher wattage one.

I would look at 75-100watt LED spotlights for that height

 

[JDFish]

I have 2 30w spotlights and im getting ALOT of algae. I had a 50w but the light was too bright i didnt like the look of it. Do i actually need really high light for plants like anubias, java fern crypts etc?

 

[Colin_T]

If the lights are on for 12 hours or less and you get lots of green algae growing on the glass, then the lights are sufficient for the tank.

You do not need bright light for Java Fern or Crypts, and Anubius is not a true aquatic plant so shouldn't be used in aquariums. If you do have Anubius it doesn't need bright light either.

 

[JDFish]

I'm going to get another 30w floodlight light and create a light bar basically. Should I maybe turn my lights on for 7 hours a day with 90w of light

 

[Colin_T]

You might be fine with 7 hours a day but you might need more. If you have Ambulia or Hygrophila polysperma in the tank, they close their leaves up when they have had sufficient light. You can watch them and as they get enough light at the surface they close those leaves up, then they systematically close up the leaves as you get closer to the substrate. Leave the lights on for an hour or two after you see them closing their leaves up.

 

[JDFish]

Only delicate plants i have are Ludwigia Arctura and Rotala Machandra

 

[Colin_T]

The only other way to see if the plants are getting enough light is to monitor the green algae growth on the glass. If green algae grows only the bottom of the glass in just a few days, then there is lots of light. If no green algae grows on the bottom of the glass (near the substrate) then the light isn't that bright.