Possible, but Ive had multiple saltwater tanks and this is the first time this happens.Well couple things, at 8 months your tank is still maturing and not stabil as you have discovered. Mine was the same but significantly better at the one year mark now. Depending on how many corals you have, a 5 gallon water change on a 100g system may not be sufficient for all of your trace elements. ICP will let you know for sure. I do 10g water change on 80g system and still have to weekly dose red sea trace elements to maintain levels. I also had to double dose neophos and neonitro when my tank was at 6 months to maintain levels. Now at one year everything is much more stable.