Is 8/3 stranded wire ok to power a 100amp subpanel?
Take that subpanel back

I know the panel is marketed as "12 circuit (6 space)".  This is bold-faced lying. This claim depends on using "double-stuff" breakers (2 breakers in 1 space) which cannot support GFCI or AFCI, required in almost every circuit today. Who cares, though?  I need 6, I have 6. True, that's fine for today.  But look how quickly you came up with this idea.  Tomorrow you could come up with another idea that needs some more spaces.  As R.M.E. discusses, you are electrically undersubscribed (especially if you go 300W for the heaters).  Or you could later upsize the supply wire. 

Swapping a panel is a nightmare job, and you pay for the panel twice.  The right time to upsize the panel is when you buy the first one.   I for one like to finish a job such 60% of the spaces are still unused.  So here, a 16 or 18 space panel would be my minimum.   Also, larger panels come with free "starter" breakers - and if a panel is $35 extra but comes with a $40 AFCI breaker, that's a win. Another win is when a panel comes with the mandatory ground bar(s) instead of chiseling you for another $8 for it.  

Subpanel requirements

I smell the influence of big-box stores here.  Buy cable staples there.  Complex things like this, go straight to a proper electrical supply house. Their staff gives good advice, most are competitive on price, and they stock a wide variety so they are able to sell you exactly the right thing.

The panel must have a separate neutral and ground bar.  The neutral bar will be tied to ground/chassis, and you'll need to remove that tie. 

The panel needs to be at least 40A obviously; larger is fine.  A 200A panel can handle 40A.  Since you're in the same building, main-lug is fine, main breaker is also OK, and the main breaker size doesn't matter. (>=40A obviously). 

AFCI/GFCI

Attics are ambiguous so you'll need to have a conversation with your AHJ (that's Authority Having Jurisdiction, or your local electrical inspector) about whether you need AFCI, GFCI or both on these receptacles.  Since I doubt you're gingerly stepping on open joists at peril of putting your foot through the ceiling drywall, I rather expect this attic is at least partially finished.  If it's finished enough to be livable space, then it definitely needs AFCI and that needs to be in the panel (or right next to it in an awkward deadfront configuration) since AFCI's main job is protect the wiring after the panel, and as such, stop house fires. 

Given the presence of water and a heater immersed in water, GFCI also seems like a pretty good idea, but that can happen at the receptacle. 

Cable

The 8/3 cable (also with ground, meaning 4 wires in total) will suffice for 40A, the breaker you already have there.  If you need to extend the cable, remember you must make splices inside a junction box!  And the junction box cover must remain accessible without tools (pulling an oven out is OK). 

Also beware the building codes: Some Codes require that you provision a 40A service to the oven location even if you aren't using it right now.  You are still OK if it is legal for you to remove the subpanel and its extra circuits and doing so won't violate any other building or electrical codes (e.g. location of receptacles).

That "conduit"

You're not using it as conduit.  You're using it as a protective shield to protect the cable from damage. That means you do not need to foll the rules of the "conduit wiring method", but do need to follow the rules of the "non-metallic cable wiring method". 

You simply cannot have data cables running with power cable. You also can't have low voltage (thermostat, doorbell) cables run with power cables.  If you are installing raceway and anticipate sending both power and data up it, install 2 raceways at least 4" apart. 

Since you're in split-phase 120V/240V power, this is simple: generally you can't have more than 4 circuits running together without having to painfully derate their capacity. Exactly 4 circuits is OK if wire size is #8 or smaller.  For larger wire, you have to get out the sharp pencil and  run the numbers.  (The derate rules actually apply to 2 circuits or more, but that's only an issue for heavy feeder.) 

Subscribing your power

First, what you have (4-wire, 40A) is 240V x 40A = 9600 VA of available service.  If you are using 120V, you have two 120V "legs" of 40A each, or 4800 VA per leg. Do everything you can to balance the circuit, overloading 1 leg will cause both legs to trip down at the main panel.  Re-read my answer on "double-stuff" breakers above for how poles are laid out inside panels, for more info on balancing.

A "continuous load" (potentially on for 3 hours or more) must be derated by 125% (i.e. a 1000W load counts as 1250W).  Only you know your setup, but I assume if a 1000W tank heater ran at full power for 3 hours solid, you'd have fish soup.  So I'm not derating those, correct this if I'm wrong.  For a resistive heater, watts = VA. 


A/C: 8A x 120V = 960VA x 125% = 1200VA
Heaters: 1000W = 1000VA ..... x3 = 3000VA (on at the same time after a power outage; otherwise intermittent). Now if the heaters are continuous loads, they count as 1250VA each. 


So your installed, known loads take 4200VA (derated) out of 9600VA available, leaving 5400VA for your two receptacle circuits.  You're fine. 

In fact, I would pencil in 500VA for each receptacle circuit, and then you're at 54% of circuit capacity.  I'm worried you have too few spaces, because this can support more. 

By the way, this sort of "oversubscribing" panels - having 95A of breakers in a panel being supplied with 80A - is totally normal at much higher ratios than this.  Use known loads if you know them, but for the loads you can't know, you pretty much count on not every circuit being used to the max at the same time. 

If, indeed, you do not need to derate the tank heaters, you may be able to get away with 2 of them (1000VA each, derated) on a 20A circuit (2400VA).  This could save you some money on AFCI/GFCI.  One tank plus the A/C (1200VA, derated) would total 2200VA and would also fit on a 20A circuit.