I agree with Boxermom. For a GSP, which is going to get to around 12-15 cm, you probably need to be looking at a tank around the 30 UK gallon mark, at least.
If money is a factor, there are workarounds. Get a big tank, but economise on the hood and the lighting. A simple condensation tray will do as a hood, and you don't need lighting at all. Putting a household lamp nearby will do fine. I sometimes place breeding tanks by windows, and that can work very well. You don't want direct light, at least not for more than an hour or two, or the temperature will rise too much. But otherwise sunlight is fine, and the fish seem to enjoy natural daylight.
Undergravel filters are another cheap but effective alternative. On the downside, they aren't easy to maintain, and setting them up can be a chore. But on the flip side, they are very good at cleaning water of nitrite and ammonium, and because you can make one from coral sand and calcum plus chips, it will also buffer the pH and hardness of the water. Read up on undergravel filters for marine tanks, and build one of those.
£40 should get you a three-foot tank with a 30 gallon capacity, which would be the minimum for a single GSP. Factor in another thirty odd pounds for an undergravel filter plate and a powerhead (ideally, get two smaller powerheads instead of one big one). Alternatively, a reasonably powerful air pump and a couple of airstones could be used instead of the powerheads. This would be cheaper but noisier. A heater should cost you under £20. To my mind, that's a much better set-up than £75 for a 15 gallon tank.
You can always add niceties like hoods and lights once you have the extra money to spend.
Cheers,
Neale
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