Aloha,
so, I'm completely new to the hobby (I did have a small freshwater community tank aaaaages ago) and might even be a little wrong here since I don't have a reef tank and won't have one, eventually. But I was hoping I could still find some help for my saltwater tank, after all
So, I'm still in the setting up phase. This translates to: my tank is still empty and waiting for the fish.
However, a friend gave me a sea urchin, already, to actually get started and have something living, while waiting. However, I'm not sure he's doing ok. The first night I got him, he moved into a hiding place and from there didn't move ONE BIT for the whole night. In the morning I checked if he's still alive and from there on he started moving. However, now he's ONLY hanging out between the filter and the opposite wall, going back and forth, and only towards the top of the tank, close to the water surface. He even went upside down (like literally!), yesterday, his mouth at the surface, attached to the side wall and slowing making it's way back. Even if I gently put him down on the sand somewhere else on the tank, he's going back to this side.
I wonder what's wrong with him and am a little worried
I have a 40 gallon breeder with a hang-on filter for up to 55g. A friend mentioned I may not have enough dissolved oxygen, but is this really a possibility? Especially since nothing has been living in the tank, before, and thus used up all the O2?
What else could it be? (Ammonia was fine, still need to check other parameters. But again, why would they be bad? Nothing in there, yet...and I'm using - ocean water)
I'm worried about this little guy - or is he just being weird? But I also wanna make sure there's nothing wrong with my tank before putting any fish in.
Thanks for any help and advice
so, I'm completely new to the hobby (I did have a small freshwater community tank aaaaages ago) and might even be a little wrong here since I don't have a reef tank and won't have one, eventually. But I was hoping I could still find some help for my saltwater tank, after all

So, I'm still in the setting up phase. This translates to: my tank is still empty and waiting for the fish.
However, a friend gave me a sea urchin, already, to actually get started and have something living, while waiting. However, I'm not sure he's doing ok. The first night I got him, he moved into a hiding place and from there didn't move ONE BIT for the whole night. In the morning I checked if he's still alive and from there on he started moving. However, now he's ONLY hanging out between the filter and the opposite wall, going back and forth, and only towards the top of the tank, close to the water surface. He even went upside down (like literally!), yesterday, his mouth at the surface, attached to the side wall and slowing making it's way back. Even if I gently put him down on the sand somewhere else on the tank, he's going back to this side.
I wonder what's wrong with him and am a little worried

What else could it be? (Ammonia was fine, still need to check other parameters. But again, why would they be bad? Nothing in there, yet...and I'm using - ocean water)
I'm worried about this little guy - or is he just being weird? But I also wanna make sure there's nothing wrong with my tank before putting any fish in.
Thanks for any help and advice
