Why are my tips going yellow?

Can you post some close up and clear pictures of exactly what it is that is concerning you please. This would make giving an informed opinion much easier and help solving your question much more specific. Generally though, burnt leaf tips are the first sign of a soil mix that has become over loaded with residual nutrients from either feeding too much or too often and sometimes both. The soil becoming over loaded can often be traced back to growers that don't water thoroughly, to the point of run off each time, which causes the nutrients to build up to toxic levels in the soil, which then leads to the leaves burning. Hope this helps, Organoman.