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Fish emulsion is a complete organic fertilizer but chlorosis on leaves are usually a sign of mineral deficiency or over watering. If you are not over watering, then you can boost the minerals by adding something like seaweed extract or azomite. If you are low in micros and the pH is not 8 then the leaf color should perk up in a very short time.
Tomatoes are heavy feeders of water and fertilizer. Any container plant must drain well. When the plants are small, they will have a lot of soil volume compared to the plant root size so you need to be careful not to overwate; especially in a plastic pot. When the plants are fully grown they need a 20 inch pot (indeterminate) as they have an extensive root system and when they are in fruit production in the peak of summer they may need to be watered more frequently, sometimes two or three times a day or you may get blossom end rot.
I don't do organic in pots, it has always caused issues for me, but other people do have good success. Some people swear by tomato tone and plant in compost. My compost has a pH of 7.8 and holds too much water so that is why I only use it to mix into my garden soil and I use an acidic fertilizer.