Fish swimming behavior (guppy and molly)

Question
QUESTION: Hi there! I poked around in the Q&A but I couldn't seem to find what I am looking for. I hope you can help.

We have a 10-gal tank with one guppy, one molly and one black skirt tetra. Everything is going fine, except I am curious about some of their (semi-repetitive) behaviors.

Specifically, both the guppy and the molly (the tetra less so) do this strange up-and-down swim sometimes. They pick a side/corner of the tank and go up and down, up and down along it. Sometimes they poke around in the gravel at the bottom and then resume their vertical "laps".

We have a bubble wand along one of the short sides of the tank. The fish sometimes do up-and-down laps through the "wall" of bubbles. To us, it mostly looks like fun! But I wanted to check whether we're right and this is them being playful or if it actually means something else.

We noticed the fish swimming vertical laps today after a water change. Could it be a sign of stress/nervousness at our interference with their habitat? Or were they enjoying the fresh new water??

Thanks for your help!
A



ANSWER: Hi Allison:  they are playing... mine love to surf the water filters water fall... the vertical water laps are probably a bit of being stressed from the water change and the excitement of having you guys near them... They are very social creatures and would be REALLY happy with two or three more of each of their species... you should be able to put three of each kind in your tank with little issue.

Hope this helps... dave

---------- FOLLOW-UP ----------

QUESTION: This is great info, thanks Dave!

We have had our tank running for only a couple months and we're definitely looking to add some more fish. I just want to confirm - do you really think a 10-gal tank can support 9 fish? At the pet store, I believe they recommended far fewer (not that i consider them experts... we've had mixed results with them).

(by the way, we also have a snail and a little ghost shrimp, and live plants and several stalks of bamboo planted in the tank. We love our little ecosystem!)

Answer
Hi again:  Yes I think a ten gallon tank can support 10 fish that are less then 1.5 inches in length... the live plant, shrimp and bamboo make it possible to add additional fish... but lets just stick with 9 and maybe a cory cat fish... they stay small... dave