Inquiry learning is an effective learning approach if learners are properly guided. Its effectiveness depends on learners' prior knowledge, the domain, and their relationship. In a previous study we developed an Experiment Design Tool (EDT) guiding learners in designing experiments. The EDT significantly benefited low prior knowledge learners. For the current study the EDT was refined to also serve higher prior knowledge learners. Two versions were created; the "Constrained EDT" required learners to design minimally three experimental trials and apply CVS before they could conduct their experiment, and the "Open EDT" allowed learners to design as many trials as they wanted, and vary more than one variable. Three conditions were compared in terms of learning gains for learners having distinct levels of prior knowledge. Participants designed and conducted experiments within an online learning environment that (1) did not include an EDT, (2) included the Constrained EDT, or (3) included the Open EDT. Results indicated low prior knowledge learners to benefit most from the Constrained EDT (non-significant), low-intermediate prior knowledge learners from the Open EDT (significant), and high-intermediate prior knowledge learners from no EDT (non-significant). We advocate guidance to be configurable to serve learners with varying levels of prior knowledge.