Bubble bath is a fabulous additive to a relaxing experience.  The fluffy white clouds that envelope, soothe and clean make the whole thing worthwhile.

As someone who makes bubble bath, however, there are a few tricks you need to watch out for.  Please note this is a "from bubble bath base" post, and not from scratch.

Bubble bath is a liquid surfactant blend that has a limited capacity for holding additional products such as fragrance or essential oils. When that capacity is reached you usually find one of two things will happen - the bubble bath will thicken up, or the base will thin out potentially to water viscosity.
Each fragrance/essential oil may react differently so testing a new scent carefully to see how it behaves is highly recommended.

If your bubble bath base thickens up with the addition of your scent the fix is easy.  Simply add some water to thin it back out.  If you need a lot of water to thin it out you may need additional preservative to compensate for the additional water.  Euxyl PE 9010 at the rate of 0.5% of the water weight only would be a good one if you need  to purchase one.  If you have a suitable preservative on hand use that at the low end of the recommended usage rate.  You only need to preserve the additional water not the whole batch (as the base is already preserved) so take some weights before you start so you can weigh again when you are done to know how much you added.

If your bubble bath base thins our and becomes too runny for your liking it can be thickened back up with salt. 
I like to add 1 tablespoon of salt to 4 tablespoons of hot water and stir until it is completely dissolved.  By making a solution rather than adding the salt directly you are ensuring it is dissolved prior to adding it to the mixture.  Dry salt can be added but ensuring it is fully dissolved can prove difficult.
Add the solution at 1% of the weight of your mix and stir thoroughly. Repeat until the correct viscosity  is achieved.  Accurate note taking though the process will give you data to use next time without having to reinvent the wheel.

PLEASE NOTE: Adding too much salt will cause the product to revert to its previous liquid state and cannot be re-thickened using this method again.