As has been mentioned Thermalite will not be strong enough.You will be needing to hold back 1 ton of water per 1000 galls, that needs steel reinforced concrete walls on a good solid base. Depending on the type of subsoil / water table etc. you may need far better substantial foundations if you want to avoid settlement cracks appearing under the weight of it all. Let us know more of finished volume, soil type, depth into ground, height and type of lining (if you ever decide to F/G it must not crack)
In my opinion the 7n type of blocks will need steel ty`s especially at the corners, on a much better foundation ring than 6″. Perhaps 16″ with rebar might be OK. Plus the concrete base around the B/Drain. If you really want walls built with blocks, you could hollow blocks with Steel tied in rebar & concrete infill (the corner tied rebar will prevent it cracking) .
As you are on a small budget, consider this;–
I built mine 4 years ago 6 feet deep into clay but in a totally different method of poured concrete as I wanted a fast, cheap build. After digging the hole (bit of help hand digging 30 tons of soil & hand filling a concrete mixer) to filling with water. This fast method of build involved bending a cage of steel reinforcing mesh and fitting around B/Drain, including all the shuttering, 100mm of Insulation, all plumbing and filters, basically on my own (excluding Fiber Glassing) all in 3 weeks and still doing my work most days.
As the ballast is so cheap my total build cost was so low with the savings, it more than paid for the cost of fiber glassing, finishing with stainless steel surrounds to cover my pond lighting and all the pond lights.
Selfbuild poured reinforcedConcrete,100mmCelotex,Fibreglassed, HybridASHP heated, Covered, B/D,Vortex,Cloverleaf 3(with K1mod) Easypod,Clarity shower/foam fractionator,Skimmer,Seive,Various Messner Eco pumps,Floor returns,Fluores&HighpowerLED lighting