We have two basins outside our house that have water lilies and tiny fish my wife calls mosquito eaters. The problem is that the water has to be topped up regularly much more than just evaporation and assume the basins are porous though I see no water on the outside. Perhaps osmosis. Can you tell me what type of sealant to use that is non toxic to the plants and fish to stop this? Something easy to get at places like homepro. It looks like clay basins.

The problem is exactly as you suspect. Evapouration and the clayware absorbing water.

A clue to absorbing is when the outside of your pot starts to go green.

We have similar fish bowls but smaller diameter and lose about 3 cups of water a day.

Yours has a bigger surface area so the evapouration would be greater.

If you really thought you were losing a lot throigh the clay then just paint a laquer inside. It dries very quick and should be safe.

The laquer will of course break down over 6 month and you will need to repeat.

But worth a try to see if it saves you topping up as regular.

You can use a non-toxic (solvent-free) epoxy paint available at most swimming pool or DIY stores. It's suitable for drinking water applications.

Multiple layers application is advised.

Just to provide a paint who is suitable for your request : the paint spec can be used as reference if you prefer another brand.

http://www.jotun.com/me/en/b2b/paintsandcoatings/products/tankguard-dw.aspx

Evaporation cycle is minimal due to ambient temperatures of +/-25 C. On the other hand your plants on top of water level will absorb water...

You also have too much duck weed on top of the water,your fish will

have problems with aeration ,especially at night ,I used a paint from Jotun

a the paint and a hardener,cured like glass,but could be expensive just

for the job you want.

regards Worgeordie

Thanks for the replies. Yes, the duck weed is a bit much and grows remarkably fast so will do something about that as I have to drain and clean it before sealing.