Just noticed this post =P... I would remove both fish and frog then co2 bomb the tank with dry ice... it is the fastest and safest way because things can morph if given the proper environment. I would be worried that the nematoads would morph into a sect that can attack the frog or fish anyway (I am way to paranoid though). Just my concern to be honest. If you have a screen top then just go to lowes and have them cut a piece that would fit perfectly and then do it =P
The nematodes are not going to morph into anything other than one of the 4 stages in the life of the nematode.
They will do absolutly no harm to the frogs by preying upon them and I have been in contact with the seller about the frogs eating an affect insect and he said it is highly unlikely that the nematode (or the bacteria with the nematode) will cause any harm to the frog. In fact he said he was 99.999% sure it would be absolutly fine.
I also asked him if the fish were in danger of coming across one that happened to make it's way in the water and he said they live on land their entire life and if submerged they will sink to the bottom and die within 12 hours.
The dry ice would have been a good idea but now having purchaced the nematodes, it's a little late. I know I will not be able to rid my tank entirely without using the CO2 method but if they can control the population than I am ok with having both the nematodes and the gnats.
I am going to let the tank set for a few weeks and then put some more springtails in the tank to help out the population. In the meantime I am looking for ways of allowing the frogs to climb in and out of the water. Because of the way the glass fits, they will not be able to just grab on to the glass and pull themselves up so I was thinink of using vine (real or fake) and sort of making it look like tree roots so they can use those to climb out. The only problem I am having is finding some vine thick enough.
If anyone has some other suggestion please share it!
Thanks!