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I have 3 tanks. 1 with my fishes and 2 other are technical. The smallest one is 0.5l and I'm using it as trash bin where I put dead plants and algae from main aquarium and it has sand from the see at the bottom. For more than 5 monthes it was full of algae and no alive creatures. Now it has many live black dots that are very fast moving and now the tank is very clean. Second technical was planned to used for daphnia (didn't get it yet). It has 5l volume now. In the bottom it has simple ground and rasied Glossotigma eletinoides from the seeds. Also it has many algae (including Spirogyra). Two tanks are closed tightly.

I have a some questions about it:

  • What kind of animals are these?
  • Can I move black dots to the other tank to have it clean from algae?
  • Are these creatures safe for me?
  • Are these creatures safe for my fishes?

I recorded video, it has 3 parts (1 tank, 1 tank in zoom, 2 tank in zoom): https://youtu.be/3jbqPiAcKOc


Update:

  • Tank 1. This animal is hard to catch Tank 1

  • Tank 2. In water I see only one type and on the stick I got 2 different animals. The left one looks like one of the species of daphnia. Tank 2


Update 2: I think I found what I have in the tank 1. It is some sort of Ostracod (seed shrimp), because I saw whiskers and slit in the shell enter image description here

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    $\begingroup$ Welcome. It's best practice to include (an) inline pictures of the critter at hand. External links are discouraged and I am personally quite hesitant to click on them (spam, phishing etc) $\endgroup$
    – AliceD
    May 3, 2021 at 14:39
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    $\begingroup$ Welcome to Bio.SE! Two recommendations to ensure a better response (to follow up form the previous comment): 1. Use a site like ezgif.com to convert a few seconds of your video to a gif to include the gif as an inline photo in this post. 2) Is there any way to get more magnified images? -- the resolution on these videos make an accurate ID tenuous at best. $\endgroup$ May 3, 2021 at 14:49
  • $\begingroup$ Looks like in the first picture it could be Pleidae or some Helotrephidae, or not?) $\endgroup$ May 3, 2021 at 16:23
  • $\begingroup$ Also I dropped fruit twig inside 1st tank and these creatures occupied it fully. $\endgroup$ May 3, 2021 at 16:41

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Yep, these are definitely ostracods. Some are predators, but their small size and preference for that plant stalk suggests that they are detritivores. In that case adding them to your fish tank will be beneficial, as they will clean the water by feeding on suspended particles, and serve as a source of food for smaller fish and animals, much like copepods or daphnia would.

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