No fertilizer, They are relatively heavy feeders. Too much light can do the same.

I would just do the tip of leaf trimming - whatever is already white - is not going to turn back to green, start doing a regular fertilizing - 1/8 - 1/4 teaspoon per gallon depending on the fertilizer - and make sure the place where it is sitting is not getting direct midday sun - or it if it is under the tubes - move it to the edge of the shelf.

I always try to keep a Strep planted as the king of the hill - so I would pull the pot off, add a couple of tablespoons of soil and put it back - to have the crowns just above the edge level.

Thank you Irina! I will try that. They only get morning sun, so I'm guessing its the fertilizer. I only have 3 streps in small pots so I don't need much fertilzer mixed up ahead of time. I guess I'll mix it in a half gallon. Do you just use miracle grow type fertilizer and use it extra diluted every time you water?

No, I use DynaGro Grow and it will be 1/4 of a teaspoon per gallon. Since it is an excellent fertilizer for indoor plants - you can use it on most of the plants you have.

Miracle Grow fertilizer is heavy on urea - and I avoid it. You can recycle a gallon jug - water, juice, tea... and just keep the fertilized water there and pour it in a small watering can every time you water. Since I have a whole hoard of plants - my approach - a 33 gallon plastic trash barrel in a spare shower, 2 tablespoons of fertilizer, fill the barrel, start the sump pump. DynaGro Grow and Fox Farm Grow Big - are liquid fertilizers you do not need to dissolve ahead of time.

There are several possibilities. First I'd check /very/ carefully for a root mealie infestation. A few years ago some of my streps were showing similarly yellow leaves and Dale Martens hopped right in and said they probably had root mealies. Sure enough, they did! May not be a bad idea to repot, washing the roots very carefully and sterilizing your pot or using a new one - and adding insect granules to your fresh mix. You should be able to see root mealies if they are thick but they may be very difficult to see and easily mistaken for perlite if the infestation is not heavy.

The other suggestions here are also good about feeding, light, temperature, etc. So check them all and make corrections as necessary.