Any reputable aquarist will tell you that as a general rule and unless you are very experienced fishkeeper, you should not keep amphibs. and fish together in the same tank.
For one thing their feeding requirements are different and for the second, amphibs. pass through a lot more harmful waste. Remember unless you keep carnivorous fish, you won't be feeding them "meat", but with amphibs. you will. So you will need a really good filtration system and change the media often.
Maybe im wrong ...but 20 gallons and u have a bala shark...well i think thats to small a tank...at least when it gets bigger....id think ur tank sounds pretty much stocked right now...no need for a frog