whats the tap water ph?
equaliser will probably increase the ph because it is designed to improve ph kh and gh in soft water by adding minerals such as calcium.
if your tap water ph is lower then your tank ph then you have something in the tank increasing the ph.
not all plants like a high ph hard water tank and prefer it more acidic.
if the axolotls are happy being in tubbed water then there is an issue with the tank.
if you can get a liquid kh gh test kit and test the water in the tank.
what are the tank water levels ie.. temp/ammonia/nitrite/nitrate
if you are using tannins then almond leaf extract can be used (the same ones that are used to make black water), anything that is used for tannins will darken/brown the water, this can be removed with activated carbon, tea bags can be used although be aware there is no information on the effects of caffeine with axolotls, normal usage is one tea bag to 10 litres water. humic acid can also be used for tannins and is a dose to taste ie.. how dark you want the water.
don't use any more bicarbonate of soda, bicarbonate of soda is used to bring the ph up, by adding more bicarb you are just increasing the ph, also because it will also raise the carbonate hardness (kh) it will resist attempts to lower the ph.