Indicative vs. Subjunctive
 

Is the indicative as well as the subjunctive correct to use in this sentence?

Keep this money in your piggy bank so you don't buy something you don't need.

Guarda este dinero en tu alcancía para que no compres algo que no necesites/necesitas.

When I spoke this sentence, I used the subjunctive, but I questioned my choice after hearing myself say it.

Any and all input is appreciated.

You are right. I was taught I learned that para que is followed by the subjunctive if para que is followed by a conjugated verb. This is like English when so that can has a subjunctive option:
so that is be done correctly

I don't think the OP questioned the use of the subjunctive after 'para que'. It looks like he was asking about the last part of the sentence where both the indicative and the subjunctive forms of the verb were given.

There are three clauses in the OP's sentence:
-guarda este dinero en tu alcancía (main clause - the subject is tú)
-para que no compres algo (secondary clause - the subject is tú)
-que no necesitas/necesites (noun clause that modifies 'algo' - the subject is tú)

If there is no change in the subject between the main and secondary clauses there isn't a need to introduce the secondary clause (no conjunction 'que' is needed after the preposition 'para,' in other words). In that case, the negated infinitive 'no comprar' would be used.

The noun clause at the end is modifying 'algo'. It is (by way of the noun clause) being categorized as something that is not needed by the other person. Those are the words.

But whether the conjugated verb in that clause is rendered in the indicative or the subjunctive mood depends on how the speaker feels about the need.

Here is where a native speaker can clarify, because this is mushy territory.

When is an unneeded item represented in the indicative mood?
When is an unneeded item represented in the subjunctive mood?

Thanks, poli and Rusty. Yes, I was asking about the last part of the sentence where I wrote both the indicative and the subjunctive forms of the verb -- necesites/necesitas.

I agree with Rusty. It depends on what the speaker has in mind.
If you say "lo que no necesitas", you may be thinking of things the person wants right now, but doesn't need, so it's better to keep the money away.
If you say "lo que no necesites", you're then thinking rather of a possible whim (something the person isn't even thinking of yet) that might take away their money, if they don't keep it safe.

Thank you, Angelica. That clears it up for me.

Glad it helped. :)