Are you sure you need 19000 Btu/hr? That's like 5600 watts: a 50 amp circuit. If you really need that many Btu's, going to the trouble of building a big tank and heating it with dinky aquarium heaters is probably not going to work well. I suspect it just won't be able to keep up. If you need a certain amount of heat energy, it doesn't matter what mechanical device you use, it still needs to be able to produce the energy.

Unless you go to something like propane heat you will be limited by that 15A circuit. Given that you are modestly insulated, I suspect you may not need as much heat as you think.

One other possibility for you is to get seedlling heat mats and set your plants on those. I use 18" X 48" mats that only use about 100W apiece, but do a good job of supplying bottom heat to the things I overwinter in my greenhouse. I set them on 1" styrofoam insulation to minimize heat loss on the bottom side.

Hi
I'm in 10 florida so probably not much help for you.
I did something similar using the outdoor lily pool and a 150 gallon aquarium. Makes a total of 1300 gallons of water circulated between 3 locations.
Used 4 600 watt standard aquarium heaters . 150 is used as a reservoir for a 6x10 foot waterfall for heat distribution. Worked fine until about 35 degrees but could not keep the heat above 50 with a low of 33 for 3 hours. Ended up shutting off the waterfall and covering the pool with bubble wrap. Fish were more important than plants lol. I think this could be tweaked but the aquarium developed an unfixable leak so is now serving as a terrarium. I was really excited about the potential of the system as the one thing it did not do was overheat like other systems I've tried.
Has many advantages at least for my location gary

The shed area is 500
ACF Greenhouses calulator does not acount for 5/8 plywood on both sides and tar paper and siding aslo does not cover foil covered bubble wrap and 1"foam board on the slab.
it's hard to come up with a heat loss value.

Help
Stan c

The ACF calculator, or any calculator, can be adjusted for situations like yours. Just figure out the R value of each of your different surfaces, the area of each surface, then 1/R = U (heat loss value), plug each of the surface areas and U values in separately and then add the BTU together at the end.

stanc-
stressbaby is correct, but, if you are not used to doing heat transfer calculations, I suspect that this might be a little complicated for you. Let me give you just a little more info. Below is a link that should give you the various R values for your setup. Where a wall is composed of several different materials, just add the individual R-values together for each material, then compute the value of 1/R and plug that number (the heat loss value) into the calculator. You will have to use the calculator twice: once for the wall surfaces, and once for the glazing surfaces, and then add the results together. The floor composition will not matter for purposes of this rough calculation. Try that and then let us know if that works for you or not.

Here is a link that might be useful: R values

Kudzu is correct, my answer was probably not adequate.
Here is a piece I wrote on "Calculating GH Costs" on another forum. I think the explanation here may be better:

Some people have asked how to calculate the cost of heating and cooling a greehouse. I hope that this thread is helpful in this regard. It is fairly technical, but I hope to explain it fully enough to allow anyone to plug in their own numbers.

First some formulas:
Watts=amps*volts
1 KWH = 3,413 BTU
1500W electric heater produces 5120BTU
Propane is 91,600BTU/gallon
Q=deltaT*A/R
Q=BTU heat requirement per hour
delta T is the difference between outside temp and desired inside temp
A= the surface are of the greenhouse is square feet, not including the floor; ie, the roof, side walls, and end walls
R= the R value of the glazing

Two important points about this formula...some heating calculators use "heat loss value" (HLV) instead of R-value. Don't get confused, the heat loss value is just 1/R. The formula using heat loss value is then Q=deltaT*A*HLV. The other important point is that you use a different temperature when calculating your average energy costs over a month than you do when you you calculate what size heater you need. Heater calculations are used to determine how much heat you will need on the coldest night of the year. For this, you use your coldest winter temps, for example the colder end of your zone. For calculating heating costs you need the average temperature for the month.

Here are some typical R-values for common glazings or coverings
4 mil polyethylene 0.83
4 mm (5/32") twinwall polycarbonate 1.43
6 mil polyethylene 0.87
6 mm (1/4") twinwall polycarbonate 1.54
6 mil poly double layer (inflated) 1.43
11 mil woven polyethylene 0.95
3 mm (1/8") glass (single layer) 0.88
16 mm (5/8") triplewall polycarbonate 2.5
Polycarbonate / fiberglass (single layer) 0.83

OK, let's say you are contemplating an ACF 10 x 12 Cottage Greenhouse. We first need to calculate the surface area of the greenhouse. Using the diminsions found on the website and some simple geometry, I calculate the surface area of each sidewall to be 55 sq ft, each roof side to be 64.6 sq ft, and each end wall to be 75 sq ft. Therefore the total area (A) is sq ft is (55*2)+(64.6*2)+(75*2)=389.2 sq ft.

The website tells me that this GH comes in 6mm twinwall polycarbonate, so the R-value is 1.54. A quick Google search shows that in this particular zone 7a, the average January temperature to be 28.4 degrees. I want to keep my greenhouse at 50 degrees. So the "delta T" is 50-28.4=21.6 degrees. Now we can calculate the BTU per hour necessary to heat the greenhouse in January. Q=389.2*21.6/1.54=5459 BTU. To figure daily use, multiply by 24 hours to get 131016 BTU/day. To figure monthly use, multiply again by 30 (days in the month) to get 3,930,480 BTU/month.

Now, lets say you heat with propane. Knowing the BTU in a gallon of propane, we can take our BTU/month and divide by the number of BTU in a gallon of propane. 3,930,480/91,600BTU/gal propane to get 42.9 gallons of propane we would expect to burn in January. We just bought propane at $1.69/gal, so this would mean my January cost would be 42.6*$1.69=$72.52/month.

Let's say you heat with electric. Take the same BTU value and instead of dividing by the BTU/gallon of propane, divide by BTU/KWH. 3,930,480/3413BTU per KWH=1151KWH. If my energy cost is 7.169cents/KWH, then heating electric should cost about $82.52 for the month of January.

Let's insulate the north wall...no sun comes through this wall, and if we insulate it with foil-backed polyisocyanurate 1" board, which has an R-value of 7.04, we raise the R-value of the north wall to 1.54+7.04=8.58.

We can recalculate Q using the same formula, but adding together the BTU lost through the north wall and the BTU lost through the rest of the greenhouse. The formula would be Q=(deltaT*A/R)+(deltaT*A/R). Our greenhouse is oriented north-south, so the north wall is an end wall with a square footage of 55 sq ft. Our BTU formula changes:

Q = (334.2*21.6/1.54) + (55*21.6/8.58) = 4826 BTU/hour

This equals a monthly use of 3,474,720 BTU, equal to about $72.99/month heating electric.

What else could we do? Well, we could add some thermal mass. Let's put 8 black 55 gallon drums in the greenhouse. One BTU is defined as the amount of heat required to raise one pound of water one degree F. The water will warm during the day and release this heat at night. One chap I know has measured the temperatures of his water at dawn and at dusk. The temp is 74F at dusk and 66F in the morning, a drop of 8F.

Eight 55 gallon drums hold 440 gallons of water which is 3520 pounds of water. A 1F drop in the water temp releases 3520 BTU. An 8F drop releases 28120 BTU overnight. Over the course of a month, this is 844,800 BTU. Therefore, the total monthly BTU could hypothetically drop further to 3,474,720-844,800=2,629,920 BTU. This is 770.55KWH heating electric. At $0.07169/KWH, this is $55.24 for the month of January.

Now on to summer costs. The greenhouse in the example will need an exhaust fan and HAF fans. First the exhaust fan. The greenhouse is 900 cubic feet in volume. Therefore, we need an exhaust fan that is rated at least 900 cfm. I found a 16" exhaust fan at Charley's rated at 1095 cfm. The website tells me it draws 1.8 amps. Remember that watts=amps*volts. So when it is on, the fan uses 1.8*120=216 watts. If this fan runs 16 hours per day in the summer, it will use 3.456KWH/day. This is 103.68KWH/month, which at the same price of energy we have been using above, amounts to $7.43/month for the exhaust fan.

Let's say you use two 8" long life HAF fans. This may be overkill for a greenhouse of this size, but you will get the idea. Each fan draws 0.6 amps, and at 120V, this is 72 watts. Running 24 hours a day, each fan uses 1.728KWH/day, or 51.84KWH/month. Total use with the two fans is again 103.68KWH/month or $7.43/month.

Nice math exercise SB :)