Fancies hanging out at the bottom of tank

[EVoyager31]

I have 4 Fancies (1 small 2”, 1 medium 4”, 2 largish 6”) in a 75G tank with 2 canisters on it (sunsun 704B and 303B), and a 3.5” air stone. I went minimal decor so they’d have plenty of space to swim, sand bottom, crinum plant, couple stem plants.

Current Parameters:

ph: 6.8

ammonia: 0

nitrites: 0

Nitrates: 20ppm (easy green for the crinum and a couple stem plants)

3 of the 4 like to just sit at the bottom unmoving regardless of water parameters UNTIL I walk in the room. Then they swim and are full of life. I don’t see anything physically wrong with them, fins aren’t clamped, scales and color look healthy, sometimes gasp but not enough for me to be worried about it.

I just think the behavior is odd, especially considering that my other goldfish tank (also house fancies) are always moving and swimming.

Do I have cause to be concerned? Are they just bored? I tried putting in a marimo moss ball once— they loved it but also shredded it and made a huge mess. 
 

Thoughts?

[Flumpweesel]

They are resting so maybe reduce the flow of the tank so they don't have to work so hard.

Goldfish especially fancies like calm waters in my experience.

I don't know anything about your canisters so I'm guessing a bit but it's the annoying thing with goldfish everyone tells you as much filtration as possible but the fish can struggle with it. 

Saying that my comets just used to sit on the gravel in the evenings next to me sometimes. (My chair was at one end of the tank I wasn't getting in there with them) 

 

[EVoyager31]

Update: I turned off the 4" air stone and it seems to have helped a TON. I replaced it with a much smaller airstone and things are going well. Thank you!