Have you done any research at all? Not asking to be snarky. Trying to discern your level of knowledge and experience.

If you haven't done much, you should do a lot of reading here on the forum. Search around. Check tank journals. All that. It will be supremely helpful.

What are you trying to accomplish? Could you be more specific about your tank and the plants you have? What kind of lighting? What will the fert situation be? All of these things will come into play when helping you figure out what you need. Though, be prepared to spend more than $100 for something decent.

 

Never been a fan of co2 art budget line but you asked.
Prob. the cheapest dual stage kit you can buy

PRO-SE Series - Aquarium CO2 Dual Stage Regulator with Integrated Solenoid

PRO-SE Series - New Generation of our most popular Aquarium Dual Stage CO2 Regulator. Completely redesigned with new pressure stabilisation construction.

www.co2art.us

 

I have an Victor HPT270C and it’s fantastic. You can get them for under $100 on eBay but your just getting the regulator. If your looking for a whole assembly setup then forget getting anything under that price.

 

Aquatek also makes cheap regulators. They do the job and were the first ones I used. I have a CO2art now and works well too.

 

I have a pair of pro-se and no complaints with them - i haven't used others so can't comment. I know some people use milwaukee regulators without complaints but i don't think the inexpensive ones are dual stage so there is always the end of tank dump issue....

 

There's a two stage airgas regulator on e Bay right now shipped for $28 (25psi max working pressure which would be fine for a reactor but not all ceramic diffusers). You'd need to pick up a cga 320 nipple and nut, solenoid, and needle valve / bubble counter. You could do that for right about $100.

 

for $100 you will need really good luck to find the good quality parts in liquidated price, and build them into a complete co2 system yourself.
the cheapest industrial grade diaphragm two stage regulator alone is about $250 in retail, normal range are $500 to $800, and stainless steel two stage are $1000 and more with pressure gauges.
but if you know what to get and willing to take the risk, you may find a used two stages regulator for around $50 (not include shipping charge nowadays), on evil Bay.
and there are right size fittings, metering valves, solenoids, they are all required good luck for good price and most important, to know what to get....

all co2 system work, custom built or commercially available systems such as aquatech or co2art, choose the co2 system according to your budget, they might have difference according to their design and in build quality, but know how to properly get them to work you will be fine, and most of the commercially available systems have good customer service to back up their products.

 

I did a lot of research on regulators several years ago. The forums are full of nightmare from people trying to save a few dollars. I skipped all the Amazon ones and bought a mid grade directly from CO2Art. Probably the one the other guys are suggesting. I installed it in 5 min. Set the flow as instructed by many on this forum and have never looked back. It just works! I have around 200 pounds of CO2 through it. 2 min tank change. Turn on pressure so Guage’s fill and turn off valve to tank. Wait an hour then if pressure is still on Guage’s there is no leak. If no pressure there is a leak. Re-read CO2Art instructions. Reinstall regulator on tank and check again after an hour . Do not over tighten or soapy water nightmares begin.

 

I went with FZone and am happy that I chose this unit over Aquatek or Fluval. I like having the flexibility to connect to CGA, Paintball or 90gram disposable CO cartridge. I connect 24oz Paintball canisters which last about 40 days on each refill. I refill at a local Sporting Goods shop for less than $6 / refill. It only comes with a working pressure gauge but this suffices for my needs. After documenting various results at different times of day over months, I found that operating at 24psi working pressure and 3 bubbles /sec rate work fine for my setup. I also use a fibst stainless steel diffusor and Hydor Koralia pump to better help the CO2 disperse through the tank instead of mainly going straight up to the surface. It is also a good idea to add an airstone to help the fish acclimate to the CO2 better.