Is a fuge a MUST in a 55g SPS tank


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Just wonderin because im about to start my new tank and I dont have a space for chateo to grow due to my tank has the in sump system, sorry its a DAS with the h39 internal sump. Im planin. In doing a 10% wc wkly. please help.
It's not necessary. A refugium would just make the tank more stable (more volume, more biological filtering), but you can definitely have SPS survive without one if you watch water parameters.
nothing is a "must" - as long as you have a working system/plan to replenish the good things and remove the bad. a fuge with cheto is a very easy way to export phosphate and nitrate and culture pods and plankton for food - but there are plenty of other methods. many of the better systems attack the same problems from multiple angles.
here are the some of the basic goals most try to achieve with their filtration systems, and a few of the tools to do so off the top of my head:
1) bulk waste/detritus removal - mechanical filtration, filter socks, sponge filters/pre-filters, vacuuming(water change), settling tanks, etc
2) fine waste and water clarity - protein skimmer, diatom filter(temporary), activated carbon or similar, bacterial dosing or cultivating(carbon dosing).
3) nitrate reduction - macro algae culture and export, bacterial dosing (with skimmer), bio-reactors, chemical additives, water changes, etc
4) phosphate reduction - macro algae culture and export, bacterial dosing (with skimmer), phosban-reactors(or similar, like ferric oxide), chemical additives, water changes, etc
5) oxygen/gas exchange - pumps. water flow and surface agitation. surface skimming.

there is lots of overlap - but i would say if you intend water changes to be one of your primary nutrient exports, you should plan on larger than 10%. it will really depend on the nutrient import and uptake. for small tanks its easy enough just to do 50-80% water changes once or twice a week, but for large tanks it can be a pain to keep nutrients low with water changes alone.
here is a nice calculator to see how water changes add up over time:
Makes sense, I am planing on adding a gfo reactor and a carbon reactor to the system, would that be any help with me just doing a 10 or15% wc? Oh forgot and a protein skimmer too. Im going to be using Tropic marin pro reef the same I use with my 25g reef I currently have.
its just a way to see how the water changes average out. so say you have a 100g tank, and you do a 10g water change each week - thats 10%. the first week you will have 90% old water and 10% pure water (the water you added after the change). the next week you remove 10g more and add back another pure 10g of water - but the water you are changing was already 10% clean - so you actually removed 9 gallons of old water, and 1 gallon of water from the previous week. so you have changed a total of 19% of the water. it helps you see how this adds up - if you do 10x10% water changes in a row, you dont get 100% of the water pure, instead you only changed 65%. which is to say if you had 100ppm of bad stuff (say phosphate.. though that would be horrible) after 10x10% water changes you would only be down to 35ppm (the non-pure number) if no more was introduced. so there are diminishing returns on multiple small water changes. in the example above if you changed 20gallons every two weeks instead of 10 gallons every week you would be down to 33ppm in 10 weeks as opposed to 35ppm using the same amount of salt mix over the same time period. but there is a sweet spot - too large or too infrequent can stress the system. also that calculator doesn't take into account additions of nutrients. here is another one that may be better to understand because it has a graph - set "initial amount of 'dirt'" and the "amount of 'dirt' in added water" to zero and put 2 in for the "daily addition of dirt" to account for feeding. the graph can give you an idea how regular water changes effect the build up if they are the only export.
Oh ok great info, I guess ill start with 15g wc weekly and test more frequently and see how that goes. Thanks alot for your help!