Duckweed turning red

Hello all.

I'm new to the hobby and got a shot at plants, but starting with floating plants as these seem much easier to succeed for a newbie like myself.

I have two tanks, a 29-gallon (6 guppies + 2 mollies + 2 neon tetras + 1 small angelfish), and a 10-gallon one (2 mollies + 3 swordtails + 1 guppy). A few weeks ago, I got some duckweed that I first placed in the bigger tank as I was still going through the cycle. Eventually, I took a little bit of it and put it in the smaller tank.

A few weeks later, I noticed that the duckweed in the bigger tank was turning red and not growing much (especially considering it is duckweed), whereas the one in the 10-gallon tank did grow quite a lot (I would say at least 4x more than what was it when it started). Some pictures below.

It's worth saying that the bigger tanks have a hang-on-back filter, and I had the duckweed constantly getting submerged with the flow of the water. But I printed some feeding rings, and they've been there for more than a week now, but still not much of an improvement. Both tanks have a lid.

What's going on with the bigger tank?

That is called Giant Duckweed. I've seen some wild azollas turning red when exposed to too much sunlight, perhaps move it to a darker corner?
Despite the name, Giant Duckweed (spirodela) is different from the most common (lemna.) duckweed
also the underside of the Giant variety is supposed to be red.

edit: there also seems to be some Water Spangles, or salvinia in the tank. that one looks to be doing well

A correction on the plant species...you have two different plants floating. One is definitely a species of Salvinia not duckweed. Salvinia auriculata is the species often associated with the aquarium plant, but several authors believe this is inaccurate. Kasselmann (2003) suggests that S. molesta is the species most often found in aquaria, and according to Mitchell (1972) is likely a natural hybrid of S. auriculata and S. biloba. S. molesta was described in 1972 by D.S. Mitchell; the species epithet is Latin for damaging or bothersome.

The other plant I agree is likely Spirodela polyrhiza, or commonly Giant Duckweed. [The true duckweed is in the genus Lemna.] You can see the differences in the photos below (they are labelled). Salvinia will grow in strings, quite distinctive.

These need good light and nutrients. Are you using any fertilizer (liquid)?

I'm not using any fertilizer, no. FWIW, I do have quite some algae still growing in the tank. I thought duckweed would utilize all the nutrients it could find, even avoiding algae from growing (although that was not my intention with it, either, but an added benefit).

My main goal is to make sure these are healthy, I don't mind the red color or the mix of different plants either; again, as long as these are healthy.

As for light, the tank does not get direct sunlight any part of the day. I do have the lid lights that cake with the tank (it's a basic TopFin kit tank), and the waterline is quite high, very close to the lights. I can try to move the plants to not be directly under the light, but I thought the more light the better.

What should I do?

There are only a couple strands of Salvinia, and they appear to be in good condition, so let's hope they are; this is a very beautiful little plant, the upper surface of the leaves will become a silvery green sometimes. The "duckweed" I've never kept, and I don't know if the excessive red is natural or a sign of light/nutrient issues, the photos I can find online don't help me much. I have seen Salvinia turn like this when literally dying off.

A good supplement that I always found necessary for any floaters is Seachem's Flourish Comprehensive Supplement for the Planted Aquarium. Another much the same is Brightwell Aquatics' FlorinMulti. A small bottle of either, it lasts a long time. Both manufacturers produce several different products under the trade name so make sure you get one of these two. They are complete and proportional, very important.

I would also try to get hold of some more substantial floating plants. Water Sprite is ideal, and there is Water Lettuce and Frogbit. Some stem plants like Pennywort do very sell floating too. These being more "substantial" are in my experience easier to maintain, and do an incredible job of sucking up ammonia/ammonium and thwarting problem algae.