Panpan - Did you test for ammonia? If you think your tests are inaccurate then you should take a sample of tankwater to your petshop and ask them to test it for ammonia, nitrite and nitrate.
If when testing for ammonia or nitrIte the levels are above 0, then you should be doing a partial 20-30%waterchange. This may mean that you should be testing either every day or every 2 days (to be on the safe side) and if needbe (ie: ammonia/nitrIte levels over 0) then do a partial 20-30% waterchange on your tank. This will keep levels safe for your axolotl.
Earthworms, live bloodworm/blackworms/tubifex worms can be fed to your axolotl.
I doubt very much that liver caused the lumps, could very possibly be a water quality problem if you have a nitrIte reading of 1.
Bellabelloo - Where do you keep your compost bin? With warmer weather they do tend to dig deeper (either into garden or further down compost bin). I moved our compost and worm bins to the shadiest part of our garden before summer set in, and managed to retain quite a substantial amount of worms thankfully. But it's always good to send the kids out wormhunting when it starts to rain (under the garden furniture, potplants, stones or bricks) - keeps them occupied and our axies happy!