Tank & stand advice

[Adrienne]

I have 120cmX45cmX45cm braced tank. If I want to use it for cichlids and all the rocks/slate that they need do I need to have a stand that has a solid top to it or can I have a metal or wooden stand that it sits into with poly underneath so that just the edges are supported.

[Warren]

A simply supported tank (metal stand supporting only the edges of the tank) requires thicker bottom glass than one with a supporting panel. In saying that, the panel used must be very stiff to have any effect. If only 12-18mm of wood is used it actually has very little effect. The deflection of the glass is quite small even when the glass is near it's maximum tensile strength. Wood bends a lot more than glass. So, the wood needs to be thick enough so it supports the load without deflection. You have 2 options:

1. Use a simple metal stand with a very thick base for the size of the tank - 25mm+ for the tank this size.

2. Add extra metal braces to the stand front to back and left to right to break the area down into smaller rectangles / squares of 200mm or so. Then a thinner wooden spreader can be used.

The technique I use for getting a good flat base works for either metal or wooden stands. Sit the stand where it will live but have it minus it's top wooden piece. Spread no more nails or a similar adhesive over the top of the base supports. Fit the top wooden panel. Sit the empty tank on some poly on top of the base before the glue sets. If the glue does not start to ooze a bit then start filling the tank a little. As soon as the glue oozes, stop and take a bit of water out of the tank. Wait for the glue to completely set before fully filling the tank.

This means the top wooden support is dead flat to the bottom of the tank and the glue takes up any unevenness in the stand. It almost guarantees even pressure between the bottom glass and base.

Using this method you will not have to worry about the weight of the rocks.

To stop point loading of the rocks you propose to put into the tank, you can add a little gravel to the bottom of the tank. This will help spread the load a bit and stop a single point touching the glass. If you don't want to use gravel then silicone will do a similar job. Squirt a bit of silicone where the rock will sit. Place the rock into the silicone. It will still be touching the glass at this time. Put a enough small packers under the rock to lift it a few mm above the glass. Once the silicone has completely set you can add the rest of the rocks. Alternatively a bit of medium hard rubber will do a similar job. The target is to stop a single sharp point touching the glass.

A common misconception is polystyrene takes up unevenness in a tank base. It will only stop point loading (eg, grit or a screw head) but will do little to nothing to help with a tank base than is not flat (eg, has a hollow or twist). In most commonly sized aquariums the surface loading on the poly is not enough for it to deform and take up the load. If it did, it would squash flat over a very short period of time. It will help with some of the high points a little but will also stress load the bottom glass panel of the tank at the same time. Get your tank base as flat as possible.

[Adrienne]

Thanks for your help Warren. I am hoping to set this tank up in the house where we live so really don't want the tank letting go. How thick would the poly for this need to be?

I have one more question for you - I already have 3 120cm X 30cm X 30cm tanks in my back shed set up on a wooden stand and the only thing under them is poly - they have water only in them and fish (of course). Am I lucky that these haven't been damaged or leaked?

[Warren]

Smaller tanks seem to get away with all manner of errors. What it means is the tank is likely to be stressed making it more likely to fail. It just lessens it's chances of survival in an earthquake or from a knock. After all, there's thousands of tanks out there with similar setups and most of them survive. What a super-flat base ensures is absolute minimum risk from the base being the cause of a problem. If this approach is taken with all aspects of setting the tank up then it is very unlikely to ever have a problem...

12mm poly should be fine.

[Adrienne]

Thanks again for your help - its this sort of advice that can not be obtained from the lfs and from websites thats so valuable to people belonging to this site.