Your turtle has a classic shell fungus infection. It won't stop growing until the habitat is upgraded. My turtle went through exactly the same thing. His habitat needed improvements in just about every area because I didn't know until I found this site what it takes to make a healthy turtle habitat. Once I made the upgrades and treated his shell for the infection, it cleared up, but it took months.
Here's a link that describes a good basic setup:
http://www.redearslider.com/startup.html
The tank needs to be much much larger and you should read up on lighting for turtles. They require both heat lights and UVB lights over their basking area or their shells and bones will get weak and they will develop shell infections.
Turtles have to bask at frequent intervals under good light to give their shells a chance to thoroughly dry out (top and bottom) and to give them a chance to bask under good UVB light. Constant moisture and poor water quality are what causes shell fungus infection. What motivates turtles to get up and bask is the heat from the heat light bulb over the basking area. How much do your turtles bask?
On the shell conditioner, it sounds like a good idea but in reality it's unhealthy for a turtle shell. Shells have pores and need to "breath". The shell conditioner is oil based and clogs the pores, which adds to the problems over the long term.
In addition, you need to take a look at turtle nuitrition. At one year, most turtles are around 4" long so yours are growing very slowly. On nuitrition, here's a link on the basics. Compare it to what your turtles are getting and if you have any questions, ask away:
http://www.redearslider.com/index_nutrition.html
Back to the shell situation - in addition to upgrading the habitat, your turtle's shell needs treatment. Since the fugus is pretty extensive, you should take him to see a vet to get some advice on treatment. In addition, here's what another good site has to say about treating shell fungus. This is from Austin's Turtle Page:
"Listed below is a regimen that I have followed in treating shell problems and has yielded great results. Keep in mind that shell injuries will take months and sometimes years to get back to looking normal if the possibility exists. It takes weeks before the shell will look like it's getting better. While treating, you are not looking for signs of healing or repair - you are looking to ensure that it is not spreading.
Clean infected areas thoroughly with a strong, undiluted betadine, iodine or Nolvasan solution. Let the turtle air dry in a warm setting for about 45 minutes.
Apply a generous coating of Silvadene cream. Work into problem areas.
Leave the turtle dry and warm, ensuring that you do not over-heat, for 18-21 hours each day.
The next day, scrub entire shell with a one of the previously mentioned solutions, using a soft-bristled toothbrush
Place them into fresh, clean water. Let them swim, drink and eat for approximately 1 - 1 ½ hours.
Repeat procedure from Step 1.
This treatment performed daily or twice daily, depending on your time allocation, has been successful in as few as 8-10 days but in more extreme cases can take a few weeks. It is difficult to tell success right away, so it is often discouraging. It is, however, quite effective and you will begin to see small improvement (in most cases) in only a few weeks."
Silvadene cream is also known as silver sulfadiazine or SSD cream. You have to get it from a vet. It's very effective.
