I moved the whole system to a better, more sunny place. It was where the table is and behind the kitchen door. I also removed the biggest strawberry plant so the others get more nutrients. I really want the weed to grow now it's finally warm.
Every Saturday I go get one or two buckets of water from the park with duck weed, the fish love it and there is some bonus little animals to hunt. I add it to where I store the rain water, where the mosquitos lay their eggs. This way I can always feed them some algaes and there is often some mosquito larvae too (besides the usual dry fish food). Balanced diet 8-)
I'm germing a new seed: Auto Moby Dick from Dinafem. (Edit 2 days later: now a picture)
@goarilla, Thanks for the tip, it usually dries up (or gets absorbed) quite quickly, but I'll definitely pay lots of attention to it in the coming weeks. I'm even thinking to fit a transparent plastic bag over them if and when forecast seems too wet for too long.