So short story I've had good growth grape culurpa sump for many years. Tank 80 gallons has been running for 12 Years. First started off with thin chaeto then the culurpa took over. Six years ago I setup a algae scrubber and the culurpa took over the screen. Had to take a knife and cut sheets off to prune. Was a great export system. Fast forward about a year ago I did a really big water change and removed back half the culurpa. In a weeks time it all died. Several months later hair algae takes over display tank its everywhere. Decided to test the nitrates few weeks back at 30 ppm. So tried reintroducing macro several times just dies off. Corals slowly started closing up several months ago now only thing doing ok is the granpallys. Two months ago I picked up on water changes 5 gallon buckets every other day. Nitrates still around 25 PPM. Notcied today cabage leather looking better. The algae scrubber is finally growing thicker green algae! Third try Cheato is hanging in there. What is going on why does the algae start growing after doing massive water changes? Also setup a 6 gallon cube plumbed to the sump and bought 5 red mangroves to see if that will help.
Any advice would be appreciated.
By sudden big water changes the nutrient balance and availability of nutrients is changed drastically, creating new battles and competition for available nutrients.
All those years you probably also have used a skimmer, removing part ( +- 30%) of TOC and DOC constantly but very selective, creating an unbalance in the availability of essential nutrients for new growth after remineralization of what is left over .
Nitrate can be removed by harvesting growth only if all other essential nutrients are sufficiently available.
In a reef aquarium a lot of trace elements may be used and stored due to calcification. In a reef aquarium a lot of trace elements may be released due to nitrification depending on the substrate on which nitrification takes place. The availability of trace elements will influence growth and is in most cases an unknown factor.
Fresh made seawater may contain a lot of trace elements, influencing growth.
Sudden water changes may create small or big pH chocks by which ammonium stored in the sediments suddenly may become available as ammonia. Also other nutrients may be released by the sediments which may influence growth dramatically.
A water change can be dangerous for an established tank with low pH and low buffer capacity.
Just a few of the many possible side effects of massive water changes.
Restoring the balance will take time. Harvesting and stimulating growth will lower the total nutrient availability. Feeding low protein food will increase the heterotrophic/ autotrophic growth ratio, lowering the nitrate production. Feeding on bentic algae also increases the C/N ratio.
What about phosphorus availability? Removing 30 ppm nitrate by harvesting growth needs 3 ppm phosphate. During periods of high growth phosphorus starvation may trigger algae and other organisms to go in survival mode, macro algae may release 50% of its biomass ( become transparent), corals may release its symbiodinium (bleaching) That is why I think phosphate is best not the growth limiting factor in an aquarium, a closed environment where the parameters for growth may change fast.
Restoring the nutrient balance by harvesting growth can be done using for the target nutrients modified feed media, such as modified Guillards F2.
