Tank Partially Empties Itself !

Funnily enough this happened to me  few days ago in the shrimpery. My filter has a piece of airline attached to it with the venturi valve on the end. I'd left it dangling outside the tank, gravity kicked in and water leaked out through the venturi valve. The valve and airline are now inside the lid and the only place the water can go is back inside the tank.

Does that help?

What must have happened is that when you did the water change, some water got into the tube. If the level of water in the tube was below the surface of the water in the tank when it was refilled, it would have siphoned water out of the tank until the level dropped to expose the other end of the venturi tube. In future when you do a water change, when you've finished turn the filter on and hold the tube straight up over the filter until the water is sucked out of the tube. But a problem could arise during a power cut where there is a risk of water seeping up the tube while the filter is off.
It should be possible to use a one way valve in the venturi tubing, but with one caution. I have a sponge filter powered by an air pump in my betta's tank and the tubing has come off the valve a couple of times. I would advise checking the seating of the tubing once a week to be on the safe side. Checking as part of a water change would be ideal.


I must admit to finding this a bit strange. I have had 2 makes of filters with venturis and the tube was only a few inches long and stuck straight up in the air. They were too short to reach the back of the tank. But I never used the venturis as I didn't like the way the bubbles just shot across the tank from the filter to the other side. I just removed the tubing and ran the filters without. Bubbles aren't necessary, though a lot of people use them because they like the looks of bubbles.

The ripples are created by the flow of water coming out of the filter. If your filter is in a separate compartment you won't be able to twist the filter to point the outlet upwards, but take it from me that they do create quite a jet. One filter I had many years ago refused to start after it had been turned off and the only way to restart it was to take the pump bit off the media bit, hold it underwater so it didn't run in air and flick the impeller. The first few times I did it I got soaked until I learned to point the outlet downwards.
The bubbles are just there to churn the water a bit more and to look pretty. If you haven't got any bubbles, the venturi isn't working so you may as well remove it. That would solve any more problems with it siphoning water out of the tank.

Thinking about it since my last post, the Eheim Aquaball and Biopower filters have this venturi attachment that does work, only I use the mini spray bar attachment instead; I only put the venturi on to see if it worked. The other filter was the Hagen Elite Mini that used to be in my betta's tank. With that one the tube kept filling with water so I never got bubbles.

Glad to hear you've sorted the problem out. I agree, it does sound a bit odd providing so much tubing; the Eheims I have that can use venturis have very short tubes.

Bettas do have a tendency to try to squeeze into small gaps. It is always safest with them to block any fish sized holes. Until last week, my betta had a cave made from a terracotta plant pot  with about a third cut off the side. The bottom was left intact and these plant pots have a hole in the bottom, so just in case, I siliconed a square of plastic craft mesh over it.