My fountain is losing lots of water and needs frequent topping up
This may or may not be normal as some water will be lost through evaporation, wind and splash.
If you suspect that your fountain is losing an abnormal amount of water the fountain up with water and leave it overnight. In the morning check for leaks from around the split bung fitting and for cracks in bowls. Leaks around stand pipes and fittings can be fixed with silicon, cracked bowls will need replacing.
If there are no leaks apparent start the fountain again and look for excess splash or possible water running back and out of the fountain, depending on the fountain this may be remedied or may be an inherent characteristic; fountains with stronger water flows from height will inevitably splash, moreso on windy days.