How many plants in how big a space you planning on running? The needs of your plants will vary quite a bit based on size of plant, temperature, relative humidity, grow style.
I use coco too in fabric pots. There is a ton of good information on this site. I'm not stating my methods are perfect, just offering information to help illustrate what I'm doing regarding water storage. Here are 2 examples you may find helpful:
Grow 1 (Oct-Feb) 6 decent sized plants in 5 gallon pots in a 4x8 space using drippers. I vegged them out for about 2 months. When they were stretching in the first 2 weeks of flower, they were consuming 3 gallons each per day. I have a 40 gallon Rez and was topping it off every other day and mixing half strength. I reset the Rez every 2 weeks except the flush which was a month.
Grow 2 (Nov-March) 3 medium sized plants in 3 gallon pots getting hand watered using DTW. I Mix up two 4 gallons buckets of half strength nutes every other day. Rez here is two 5 gallon buckets. Peak consumption was 1.75-2 gallons each per day, now its 1 gallon for flush.
I've found it easy to setup a 50 gallon water only reservoir fed by a RO filter with a float valve. I am using a heavy duty Rubbermaid garbage can on a wheeled coaster. Anything Rubbermaid is food grade plastic.
