In the past i've put a plastic tib, or a smaller plastic tank (like one of the little critter keepers) into the tank buried in the subtrate like this:

It's such an easy way to keep ontop of keeping water clean for frogs that prefer just to sit or float in it rather than swim (like pacs or FBT) just lift it out and pour it away and replace.

Or you can cut a piece of glass or Acrylic to fit and silocone it in. I used and Acrylic sheet (bought off ebay i think) and ome aquarium silicone that any pet store with a decent aquatics section should have. I imagine this method would be better with tanks that need filtration ect but i cant vouch for it. I just use a syphon/gravel vac to change the water.

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