Avocado plant

Hi everyone ! I would really appreciate if someone could help save my avocado plant as I have now tried everything! I have grown my own seed and already potted it. It seems to be doing wonderfully with big green leaves until the new ones start to come in, and then one by one my older leaves start to get brown and brittle until they die. It has been happening to all of them and I don’t know what I’m doing wrong. I heard it could be salt accumulation, so every time I water it I do it thoroughly so that it drains well. The plant has always been in place with great light, and even more so now, as I attempted to give it even better conditions. I have also repotted it, in case it was too much and clamped together. I’m watering it maybe once a week as every time I’m doing it, I water it well. I’m currently not feeding it as I’m afraid it will make it worse. Maybe the pot mix is wrong ? I’m using mature plant compost no 3 mixed with a little perlite. I’m using only filtered water also.