Interesting, I researched it an apparently it does effect fry
"generally, species need a minimal threshold intensity to be able to develop normally and grow. This is probably related to the aptitude to localize, catch and ingest prey. Light is also indispensable for body pigmentation, an important phenomenon involved in early development and growth. Too intense light can be stressful or even lethal. A few species are able to develop and grow at very low intensities or, sometimes, in the absence of light. Generally, long daylength improves larval rearing quality. The synergistic effect of `food availability-daylength' appears to be determining at this stage. In older fish, there is very little information about the influence of light `quality' but more about intensity and much more about photoperiod. Light intensity effects are not so clear and depend on the species and the experimental procedures: it is probably not an important factor for growth stimulation"
And another one who tested the effects of continuous light on growth rates in rabbitfish fry
"The effects of 24-h lighting on feeding, growth and survival of first-feeding larvae of Siganus guttatus were tested. Continuous light gave a higher mean survival (31.69%) with a maximum of 40.50% compared to natural daylight (17.10%). Larval size on day 7 also showed that continuous lighting gave improved results. Although feeding incidence increased with age of the larvae, a higher incident rate was observed for larvae held in continuous light from day 3."
I love plants, and my fry always have LOTS of plants in their tank. I always have Plant lights, in the blue spectrum for all my tanks. I use light fixtures bought from lowes. The fixtures and the bulbs are ALOT cheaper. :) Now these are not intended for use on aquariums. So when I first get them, I Tape up the plastic parts where the bulbs are attached and paint the rest with a rustoleum spray paint. Now occasionally you will need to redo this. If you are really handy with stuff, you can disassemble the fixtures and paint them inside and out for they will rust on the inside as well especially if you have a "fish room" where the moisture in the air is high. I hang them from chains about 6 inches from the top of the tanks. use a chain that a popsicle stick can fit though the links. I leave the light on continuously until the Daddy gives up the nest. I use to leave a night light on to give a slight bit of light. But I always noticed that Dad would slightly posture whenever there was the slightest bit of movement in the room. These fish did not do this in the day time. I also noticed that if my male ate the fry, I always found him bloated in the corner in the morning. Almost every time he ate the fry during the night. But I was always worried that Dad would be too stressed with light all the time. But, I took what I figured was the lessor of two evils and leave the lights on, but at night, I flip the light fixture, and place popsicle sticks in the links to hold the light upside down. This still gives quite a bit of light, yet it is more indirect.
I had never thought that it may be good for the fry, I was always worried it would be way too much and stress them. When I use to turn the lights off at night, if it was still dark out, and no natural light in the room in the morning when I wanted to turn the lights on, I would turn a smaller light on in the room, wait awhile, then turn the lights on over the tanks. This way the fish weren't woke with a "start", a shock. I noticed that every time, the fry would be on the side of the tank closest to the light. I always thought they felt safer or something, but maybe they were just doing what they instinctively knew was best for them.
So that is the way I have the set up, and thank you for bringing up this question! Here all this time, I thought it was harder on the fry, but better for dad, and really I was accidently helping! I feel better about my decision to do it now, and can stop debating it with myself.lol
At Lowes, if you look on the package, there will be a diagram of the spectrum for that particular bulb. It will even till you which bulbs are best for your type plant. Like Blue is better for green lush growth, while red, the other end of the spectrum, is better for blooms. There are some underwater plants that bloom, and they are beautiful! But for the fry tanks we just want to give them a place to hide out, inforsium to eat, and help with the oxygen and help with the nitrates and nitrite levels, even though water changes should help, there are those days that you just do not get to it. Also, the light helps keep down the plant debris.
Since I spawn using a more natural method in soil based heavy planted tanks-full to the top with water-I stay on my regular photoperiod when spawning, incubation of eggs, wiggler stage to free swim stage of the Betta fry. My photoperiod is- 10h/day in the cooler months and 12h/day in the warmer months and have not seen any difference regarding hatch rate and/or survival rate of the eggs/fry with my PP- using 6500k 20-40w florescent lights.
When I spawn Neon tetra and a couple of other species-I have found that PP and light intensity requirement to be different and will need to keep light limited if not absent during incubation of eggs up to several week after hatch.
my first couple spawns, while the male was still by the nest, and for the first week, i kept the light on all day. maybe ill switch to a 12 hr light/ 12 hr dark. or maybe even a 14/10 . deffinately something i want to experiment with.