serious questions

Yes they will use more water and nutes in flowering. They are really putting effort in to try to reproduce (hopefully you have no males around for pollination though).
As for your CO2, it should be fine. Carbon dioxide in pure form has a higher molecular density then the mean density of the gases that compose air, hence CO2 sinks.

 

How many plants? How big are they? What is the temp in the room where your res is?

 

Yes, with 12 full size plants they could easily drink that much.

When I did soil, by that point in flowering, in 3 gal pots, I was at a gallon a day... per plant.

I wouldnt bother to check for leaks, if you leaked 15 gallons of water, you would know

I would check your temps and humidity, there is a chance you are too "dry" and hot.

 

Using alot of water shows that your plants metabolic processes are functioning well. With CO2 added to the room bud production increases but not without alot of light. Also air temp must be increased to stimulate a faster metabolism on the plant (I try to keep 82-86). However the plants need to keep their internal temp lower so they use more water. I would move the res out of that room. You want you res water temp in the mid to upper sixties to compensate for high air temps. If you do this you should also see increased growth rate in your plants due to a healthier root zone. This may also help with humidity because a big ol bucket of water in a hot room makes for high humidity. Humidity should be between 45%-56%. If you still have humidity issues a dehumidifier will solve that but will also add heat so most people go with a dehumidifier and a/c combo. If you are renting and can't go crazy don't sweat it if you can get close you'll still have good results. Just remember attention to detail regarding the basics is way more important that tricks and magic nutes.

 

Sorry, didnt realize you were celsius. Ok 30 is a good air temp to stimulate the metabolic rate, but if the res is in the same room it's gonna equalize to the same temp. You want you res water temp to be more like 17-20. If you are using alot of water, topping off you res with cold water everyday might be enough to bring down the temp. If not try frozen bottles. Fill a 1 liter soda bottle half way with water, freeze it with the cap off, replace the cap and tie a string around the neck. Hanging it in the res will bring the temp down. Just be careful not to go too low or you'll shock the roots. But ideally you will want to move the res to save yourself all the hassle. Remember warmer water holds less oxygen which is needed for a healthy root zone.

 

I'd hang it just for convenience, but I guess you'll have the same result either way. Try one bottle first and add more if needed. Remember not to low. Mid eighties is good for room temp as long as your CO2 is between 1,200-1,500 ppm. After 90 deg. Growth will slow to a crawl no matter how much light and CO2 you have. At 95 growth stops completely as your plants will go into survival mode and shut down cellular production. Temps, lighting, humidity, CO2, oxygen at the root zone and pH. These are your main environmental factors. Remember these are way more important than your nutrients cause if their out of wack no nutes in the world can help cause your plants just won't use them.

 

By the way, that's a great pic. With a little tweaking you should do very well.

 

Do you have any closets on the other side of the walls of your grow room. If so you could put the res in the closet and just put the fill and drain hoses through the wall. Looks like you got a C.A.P. ebb and grow so the hoses are only 5/8". Small enough that a quick swipe of joint compound would patch it easily. Besides, if your landlord did come in and look you'd probably have bigger problems than those two little holes, unless of course he partakes. Just be careful not to hit any wiring cutting those holes. Oh yeah, you will also need a third hole and extension cord for the fill pump. By the way, where is your air pump?