My Water Is Soft!

I've just been in my water suppliers website and it says my area has soft water. Do I need to do something to alter it?

It says on the water website 20 mg/ litre calcium / 50mg/litre calcium carbonate and 3.5 clarke. I was hoping to keep some tetras and other similar little fish and a couple of dwarf frogs

That measurement is for GH (general hardness) not KH (cabonate hardness) even though they mention it terms of calcium carbonate. GH is what fish profiles use when they say a particular species needs hard or soft water and is a measure of divalent metal ions, which in the case of the water supply is almost all calcium with a bit of magnesium. GH is more important than pH; it is better to match fish to the hardness even if the pH is not ideal. I have softish alkaline water and I can keep soft acid loving fish better than hard alkaline loving fish.
KH is a measure of how stable the pH is likely to be. If there's not much KH, it gets used up quickly and the pH can fluctuate which is not good for the fish.
And for cycling KH needs to be reasonably high to encourage the bacteria to multiply. If the KH is very low it can be raised artificially for the duration of cycling though this method can't be used when there are fish in the tank.

The pH - have you tested the water straight from the tap or the water in the tank? Ideally, you need to check both, but for the tap water let it stand for 24 hours before testing. A lot of water companies add carbon dioxide and this lowers the pH. It gasses off when the water stands and that then gives the true pH of the tap water.

A lot of tetras are from the Amazon and its tributaries and need soft water. Your water is ideal for them.

But a word of caution about frogs. They are quite tricky to keep with fish. They are almost blind and find their food by smell - and by the time they've found it, the fish have eaten it all. I had two in with a betta and I had to go to great lengths to construct a feeding place that the frogs could get into but the betta couldn't.
Despite what you will read, frogs should not be fed exclusively on bloodworm. Their staple diet should be specialised frog food such as Zoo med frog and tadpole food.