yellow leaves on seedlings and fertilizing
I have a few dozen tomato seedlings under fluorescent lights inside. They are in their first potted-up stage and are in 3" peat pots and yogurt cups. The soil is a mix of sterile seed starter and sterile potting soil. I have fed them with a 1/2 strength fish emulsion/liquid seaweed mix. After they were potted up, they grew well, but now they are showing some yellowing leaves and some purple on the leaf undersides. I am concerned. Some of what I have read says this may indicate not enough phosphorus, but I am not sure. I water and then let the soil dry out, which is accelerated by the peat pots which wick away moisture very quickly. I probably wouldn't use the peat next year.
Some of the plants could use another repotting into 6" pots, but I don't have any this size. They've been hard to find. Also, if I repotted all the tomatoes into larger pots, I won't have enough room for all of them under the lights. I do have the option of a small, portable plastic greenhouse (plastic film over two racks) that a friend gave me. I could use this outside during the day and take it in at night. This is big enough to house all the tomatoes and would not require the fluorescent lights.
What is the best course of action? Is 1/2 strength fish emulsion too much/not enough? These plants need at least two more weeks before they can go outside. Thanks.