Does too much light cause stress for fish in fresh water aquarium?

Question

I have a 160 l (ca. 42 US gallons, sized 100 cm x 40 cm x 40 cm). This is fresh water aquarium.

The tank was for couple of months left almost without control. I was able to change water only every few weeks. To avoid algae I did not turn on the lights. The heater was broken so the temperature was about 21 °C (ca. 70 °F). The plants are: one large Anubias (barteri, probably) and some minor ones of unknown species.

In the tank there are only two black tetras (Gymnocorymbus ternetzi) and one neon tetra (Paracheirodon innesi).

I understand it all was not good and I agree with all criticism from you.

Now I have more time and wish to repair everything I made wrong as quickly as possible, of course, I don't want the changes cause stress to tank inhabitants and I want to do anything step-by-step, so I will control what I change and what effects it makes.

I also know that there are too few fish of both species and I am going to buy more, but this will be the last step.

At first I started with turning on the lights, it is now 5 hours a day (4 PM to 9 PM). The tank is far from window and there is no possibility of direct sunlight. As I expected, the algae begun to grow, but I am removing them by hand and their increase seem to be slowed down. The plants began to grow.

I have noticed that all three fish were not moving, they just were "hanging" in one place (this was also the same when I couldn't take care of tank). They were not hiding and they seem to look healthy. After some googling I bought a new heater and increased the temperature to ca. 25 °C (ca. 77 °F). The fish behaviour did not change.

Then I noticed that if the light turns off, the fish start to penetrate the aquarium. This made me think that the fish are in some way afraid of lights (with actual temperature of 25 °C, so this helped), however, they have many hiding places (under the anubias, there is a coco-nut and a quite large root), which they do not use. They are just hanging in one position, about 5-10 centimetres over the bottom, they move their fins. I am thinking maybe they are blinded. About 10 minutes after turning off the lights they start to swim.

I was thinking about buying more plants to create some shadow, but I don't know if it is necessary.

However, maybe the problem has its reason that there are too few fish of each species and they don't feel comfortable. I however think (maybe I'm wrong) that I should ensure that the environment is OK before I start introducing new fish.

What is your opinion?

I have measured the water parameters and they are:

• pH = 6.4
• NO₃^- = 10 mg/l
• NO₂^- = 0 mg/l
• dGH = 8
• dKH = 6 to 10
• Cl₂ ≈ 0.

One of the lamps is Hagen Aqua-Glo T8 30 watt, unfortunately I can't tell the other lamp, it is different (similar power, however), but I've lost data.

Answer 1

Are you turning the lights on and off by hand? If so, I'd strongly recommend a timer, they cost around $10 USD on Amazon, or your favorite pet store. I personally think that inconsistent light times are far more stressful than the grade of light you have. It may take a few days (1-3) for fish to adjust to any lighting schedule you set.

However, maybe the problem has its reason that there are too few fish of each species and they don't feel comfortable. I however think (maybe I'm wrong) that I should ensure that the environment is ok before I start introducing new fish.

Yes. Tetras are community fish, they like to hang out with others of their same species. Aquarium enthusiasts advocate having a minimum of 2-3 of each.